U.S. patent number 7,816,439 [Application Number 11/796,483] was granted by the patent office on 2010-10-19 for structures incorporating polymer-inorganic particle blends.
This patent grant is currently assigned to NanoGram Corporation. Invention is credited to Yigal Dov Blum, Christian C. Honeker, Nobuyuki Kambe, David Brent MacQueen.
United States Patent |
7,816,439 |
Kambe , et al. |
October 19, 2010 |
Structures incorporating polymer-inorganic particle blends
Abstract
Polymer-inorganic particle blends are incorporated into
structures generally involving interfaces with additional materials
that can be used advantageously for forming desirable devices. In
some embodiments, the structures are optical structures, and the
interfaces are optical interfaces. The different materials at the
interface can have differences in index-of-refraction to yield
desired optical properties at the interface. In some embodiments,
structures are formed with periodic variations in
index-of-refraction. In particular, photonic crystals can be
formed. Suitable methods can be used to form the desired
structures.
Inventors: |
Kambe; Nobuyuki (Menlo Park,
CA), Honeker; Christian C. (Woodside, CA), Blum; Yigal
Dov (San Jose, CA), MacQueen; David Brent (Half Moon
Bay, CA) |
Assignee: |
NanoGram Corporation (Milpitas,
CA)
|
Family
ID: |
26769976 |
Appl.
No.: |
11/796,483 |
Filed: |
April 27, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070208123 A1 |
Sep 6, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10083967 |
Feb 25, 2002 |
7226966 |
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60309887 |
Aug 3, 2001 |
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Current U.S.
Class: |
524/432; 524/428;
524/418 |
Current CPC
Class: |
B82Y
20/00 (20130101); G02B 1/045 (20130101); G02B
6/1225 (20130101); G02B 6/1221 (20130101); G02B
1/02 (20130101); Y10T 428/2982 (20150115); G02F
2202/32 (20130101) |
Current International
Class: |
C08K
3/18 (20060101) |
Field of
Search: |
;524/432,418,428 |
References Cited
[Referenced By]
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Watanabe et al., "Polymer arrayed-waveguide grating multiplexer
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|
Primary Examiner: Cain; Edward J
Attorney, Agent or Firm: Dardi & Herbert, PLLC Dardi;
Peter S.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This patent application is a divisional of U.S. patent application
Ser. No. 10/083,967, filed on Feb. 25, 2002, now U.S. Pat. No.
7,226,966, which claims priority to U.S. Provisional Patent
application Ser. No. 60/309,887 to Kambe et al., filed Aug. 3,
2001, entitled "Index-Engineering With Nano-Polymer Composites,"
incorporated herein by reference.
Claims
What we claim is:
1. An optical structure comprising a polymer-inorganic particle
blend comprising a polymer, first inorganic particles comprising a
material having an index of refraction of at least about 2.5 index
units, and inorganic phosphor particles.
2. The optical structure of claim 1 wherein the polymer-inorganic
particle blend comprises a polymer-inorganic particle mixture.
3. The optical structure of claim 1 wherein the polymer-inorganic
particle blend comprises a polymer-inorganic particle
composite.
4. The optical structure of claim 1 wherein the polymer
inorganic-particle blend comprises inorganic particles comprising
metal/metalloid oxides, metal/metalloid nitrides, metal/metalloid
sulfides or combinations thereof.
5. The optical structure of claim 1 wherein the polymer-inorganic
particle blend comprises a polymer selected from the group
consisting of polyamides (nylons), polyimides, polycarbonates,
polyurethanes, polyacrylonitrile, polyacrylic acid, polyacrylates,
polyacrylamides, polyvinyl alcohol, polyvinyl chloride,
heterocyclic polymers, polyesters, modified polyolefins,
polysilanes, polysiloxane (silicone) polymers, and copolymers and
mixtures thereof.
6. The optical structure of claim 1 wherein the polymer-inorganic
particle blend comprises no more than about 5 weight percent
inorganic particles.
7. The optical structure of claim 1 wherein the polymer-inorganic
particle blend comprises at least about 25 weight percent inorganic
particles.
8. The optical structure of claim 1 wherein the inorganic particles
have an average primary particle size of less than about 100
nanometers.
9. The optical structure of claim 1 wherein the first inorganic
particles have essentially no primary particle with a diameter
greater than about 4 times the average primary particle size.
10. The optical structure of claim 1 wherein the first inorganic
particles have a distribution of primary particle sizes such that
at least about 95 percent of the primary particles have a diameter
greater than about 40 percent of the average diameter and less than
about 160 percent of the average diameter.
11. The optical structure of claim 1 wherein the first inorganic
particles comprises rutile TiO.sub.2.
12. The optical structure of claim 1 wherein the inorganic phosphor
particles comprise Y.sub.3Al.sub.5O.sub.12 or
BaMgAl.sub.14O.sub.23.
13. An interconnected optical structure comprising a first optical
channel, a second optical channel and an optical interconnect
optically connecting the first optical channel and the second
optical channel, the optical interconnect comprising a
polymer-inorganic particle blend.
14. The optical structure of claim 13 wherein the first optical
channel and the second optical channel have different
indices-of-refraction and wherein the optical interconnect
comprises an index-of-refraction intermediate between the
indices-of-refraction of the first optical channel and the second
optical channel.
15. The optical structure of claim 14 wherein the optical
interconnect has a gradient in index-of-refraction to provide a
monotonic change in index-of-refraction from the first optical
channel to the second optical channel.
16. The optical structure of claim 15 wherein optical interconnect
comprises a plurality of polymer-inorganic particle blends located
adjacent each other along an optical path and wherein the gradient
in index-of-refraction comprises a step-wise change in
index-of-refraction.
17. The optical structure of claim 16 wherein the plurality of
polymer-inorganic particle blends comprise at least two blends that
differ by particle loading.
18. A method for producing an interface between two optical
materials differing in value of index-of-refraction between each
other by at least about 0.005, the method comprising implementing a
self-assembly process with a polymer/inorganic particle blend to
form a first optical material and locating a second optical
material in contact with the blend to form the interface.
19. The method of claim 18 wherein the second optical material is
placed on a surface prior to the self-assembly wherein the
self-assembly process contacts the particle-inorganic particle
blend with the second optical material.
20. The method of claim 18 wherein the second optical material is
deposited after the self-assembly process to form the interface.
Description
FIELD OF THE INVENTION
The invention relates to structures formed with polymer-inorganic
particle blends, including polymer-inorganic particle composites
with bonding between the particles and the polymer. The invention
further relates to processing approaches, such as self-assembly,
for the formation of structures from polymer-inorganic particle
blends. In addition, the invention relates to devices formed from
the polymer-inorganic particle blends, in particular optical
devices, such as photonic crystals.
BACKGROUND OF THE INVENTION
Advances in a variety of fields have created uses for many types of
new materials. In particular, a variety of chemical powders can be
used in many different processing contexts. Inorganic powders can
introduce desired functionality in various contexts. Similarly,
polymers can be used to form a variety of devices in many fields.
Various polymers are available to provide desired properties and/or
functionalities for the appropriate application as well as
providing versatility in processing.
Furthermore, technological advances have increased interest in
improved material processing with strict tolerances on processing
parameters. As miniaturization continues even further, material
parameters will need to fall within stricter tolerances. Current
integrated circuit technology already requires tolerances on
processing dimensions on a submicron scale. The consolidation or
integration of mechanical, electrical and optical components into
integral devices has created further constraints on material
processing. Composite materials can be used to combine desirable
properties and/or processing capabilities of different materials to
obtain improved materials and performances.
An explosion of communication and information technologies
including internet based systems has motivated a world wide effort
to implement optical communication networks to take advantage of a
large bandwidth available with optical communication systems.
Optical communication systems incorporate optical fibers for
transmission and may include, for example, planar optical
structures for manipulating optical signals in a smaller footprint.
Formation of optical devices has been based alternatively on
polymers or on inorganic materials, such as silica glasses.
SUMMARY OF THE INVENTION
In a first aspect, the invention pertains to an optical structure
comprising an interface between a first optical material and a
second optical material each of which comprises a polymer. The
first optical material comprises a polymer-inorganic particle
blend, wherein the blend comprises inorganic particles that, when
isolated, are electrical insulators or electrical conductors.
In another aspect, the invention pertains to a structure comprising
an interface between a first material and a second material each of
which comprises a polymer. The first material comprises a
polymer-inorganic particle composite. The composite comprises
inorganic particles that are electrical semiconductors or
electrical conductors, and the inorganic particles have an average
particle size of no more than about 1 micron.
In a further aspect, the invention pertains to a material
comprising a polymer-inorganic particle blend. The blend comprises
inorganic particle that are electrically conducting, and the blend
is transparent to visible light at a thickness of 100 microns.
In an additional aspect, the invention pertains to a reflective
display comprising liquid crystal dispersed within a
polymer-inorganic particle blend. The polymer-inorganic particle
blend is an optical material.
Furthermore, the invention pertains to an interconnected optical
structure comprising a first optical channel, a second optical
channel and an optical interconnect optically connecting the first
optical channel and the second optical channel. The optical
interconnect comprises a polymer-inorganic particle blend.
Also, the invention pertains to a periodic structure comprising
approximately periodic index-of-refraction variation. The structure
comprises a first polymer-inorganic particle blend and a second
optical material interspersed between regions with the
polymer-inorganic particle blend. The second optical material is
selected from the group consisting of a second polymer-inorganic
particle blend, a polymer and a non-polymer inorganic material.
In other aspects, the invention pertains to a photonic crystal
structure comprising a periodic array of a polymer-inorganic
particle blend that is interspersed with an optical material.
In further embodiments, the invention pertains to an optical
structure comprising an interface between a uniform optical
inorganic materials and an optical polymer-inorganic particle
blend. The blend comprises inorganic particles that are electrical
insulators or electrical conductors.
In addition, the invention pertains to a display device comprising
a layer of an optical polymer-inorganic particle blend that forms a
visible portion of the display. The blend comprises inorganic
particles that are electrical insulators or electrical
conductors.
In additional aspect, the invention pertains to an optical device
comprising a polymer-inorganic particle blend wherein the blend
comprises inorganic particles that exhibit non-linear optical
properties.
Furthermore, the invention pertains to a light absorbing device
comprising a first electrode and a polymer-inorganic particle blend
arranged in a periodic structure. The periodic structure absorbs
electromagnetic radiation at a desired frequency.
Also, the invention pertains to an electromechanical structure
comprising a pair of electrodes and a polymer-inorganic particle
composite. Application of a voltage to the electrodes results in a
deflection of a portion of the electromechanical structure.
In other aspects, the invention pertains to a method for producing
an interface between a first material and a second material with
each material comprising a polymer and with at least one of the
materials comprising a polymer-inorganic particle blend. The method
comprises coextruding a first optical material in contact with a
second optical material to form an interface between the first
material and the second material.
In further aspects, the invention pertains to a method for
producing an interface between a first material and a second
material with each material comprising a polymer and with at least
one of the materials comprising a polymer-inorganic particle blend.
The method comprises spin-coating the first material on top of a
layer of the second material to form an interface between the first
material and the second material. The first material does not
dissolve the second material in the time frame of the spin coating
process.
In another aspect, the invention pertains to a method for producing
an interface between two optical materials differing in value of
index-of-refraction between each other by at least about 0.005. The
method comprises implementing a self-assembly process with a
polymer/inorganic particle blend to form a first optical material
and locating a second optical material in contact with the blend to
form the interface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic perspective view of a planar interface
between a polymer-inorganic particle blend and a second
material.
FIG. 2 is a schematic perspective view of an interface along an
edge between a polymer-inorganic particle blend and a second
material.
FIG. 3 is a schematic perspective view of a structure with various
interfaces between three materials.
FIG. 4 is a perspective view of a representative waveguide
structure.
FIG. 5 is a top view of a planar optical structure with an optical
coupler/splitter within the structure noted with phantom lines.
FIG. 6 is a side view of the optical structure of FIG. 5.
FIG. 7A is a sectional view of an interconnect with a step-wise
change in index-of-refraction.
FIG. 7B is a sectional view of an interconnect that tappers between
a first waveguide and a second waveguide.
FIG. 8 is a sectional view of a polymer-inorganic particle blend
used as an optical adhesive to connect two optical channels.
FIG. 9 is a sectional side view of an optical channel with a
bend.
FIG. 10 is a sectional side view of a reflection-type,
polymer-dispersed liquid crystal display device with two display
elements shown with the section taken through the center of the
elements.
FIG. 11 is a sectional side view of a tunable vertical cavity
surface-emitting laser incorporating a micro-electromechanical
system with the section taken through membrane mounting post of the
micro-electromechanical system.
FIG. 12 is a top view of an arrayed waveguide grating.
FIG. 13 is a top perspective view of an optical structure with
three optical switches formed from polymer-inorganic particle
blends.
FIG. 14 is sectional side view of the optical structure of FIG. 13
with the section taken along line 13-13.
FIG. 15 is a sectional side view of a cross-connect optical switch
with the section taken through the switch elements.
FIG. 16 is a perspective view of a structure with a periodic array
of bars of polymer-inorganic particle blends.
FIG. 17 is an optical structure with a periodic array of higher
index-of-refraction material within the optical structure.
FIG. 18 is a perspective view of an optical structure with a two
dimensional array of higher index-of-refraction material, which can
be a portion of a larger optical structure.
FIG. 19 is a perspective view of an optical structure with a three
dimensional array of higher index-of-refraction material, which can
be a portion of a larger optical structure.
FIG. 20 is a top view of an optical structure with a step-wise
gradual change in index-of-refraction to form a periodic variation
in index-of-refraction.
FIG. 21 is a plot of index-of-refraction as a function of distance
for the optical structure of FIG. 20.
FIG. 22 is a sectional side view of a tunable optical filter with
the section taken through the tuning electrodes.
FIG. 23 is a sectional side view of a laser with two Bragg gratings
as partial mirrors.
FIG. 24 is a perspective view of a laser pyrolysis apparatus used
in the production of titanium oxide.
FIG. 25 is a cut away side view of the laser pyrolysis apparatus of
FIG. 24.
FIG. 26 is a sectional view of the laser pyrolysis apparatus of
FIG. 24 taken along line 26-26 of FIG. 24.
FIG. 27 is a plot of three x-ray diffractograms for each of three
different TiO.sub.2 powder samples.
FIG. 28 is a transmission electron micrograph of representative
titanium oxide nanoparticles formed by laser pyrolysis.
FIG. 29 is a plot of an absorption spectrum in arbitrary units as a
function of wavelength for a 0.003 weight percent dispersion of
TiO.sub.2-3 in ethanol.
FIG. 30 is a plot of an absorption spectrum in arbitrary units as a
function of wavelength for a 0.003 weight percent dispersion of a
commercial brand of TiO.sub.2 in ethanol.
FIG. 31 is a plot of refractive index as a function of particle
loading for titanium oxide nanoparticles in poly acrylic acid.
DETAILED DESCRIPTION OF THE INVENTION
Versatile materials and structures can be formed from blends
comprising polymers and inorganic particles. In particular, a
polymer-inorganic particle blend can be combined with another
material, which may or may not be another blend, to form structures
with appropriate interfaces between the different materials.
Inorganic powders and corresponding polymer-inorganic particle
blends can be used in the production of devices, such as flat panel
displays, electronic circuits, optical and electro-optical
materials, optical devices and integrated optical circuits. In some
embodiments, the polymers and the inorganic particles are
chemically bonded to stabilize the resulting composite. For the
formation of optical materials and optical structures, the
polymer-inorganic particle blend materials have optical properties
based on the components of the optical materials. In general, by
selecting the compositions and particle loadings, the properties,
such as optical properties, of the blend can be correspondingly
selected. Desirable optical structures may involve interfaces
between the polymer/inorganic-particle blends and another polymer
material, such as another polymer-inorganic particle blend, or a
uniform inorganic material, such as an optical glass. Specifically,
optical structures generally involve interfaces between optical
materials with differing optical properties, e.g.,
index-of-refraction. Various processing approaches can be used
effectively to form desired optical structures. Optical and
non-optical devices can be formed advantageously that incorporate
the polymer-inorganic particle blends. Some structures of interest
have a period variation in materials with one or more of the
materials being a polymer-inorganic particle blend.
Polymer-inorganic particle blends can be used to engineer
processable materials with wide ranges of properties, such as
index-of-refraction. In addition to having versatility with respect
to functional properties, polymer-inorganic particle blends can
have desirable mechanical properties, such as durability. A range
of polymers are suitable for incorporation into the composites,
including both organic polymers and inorganic polymers, such as
polysiloxanes. The inorganic particles generally include metal or
metalloid elements in their elemental form or in compounds.
Specifically, the inorganic particles can include, for example,
elemental metal or elemental metalloid, i.e. un-ionized elements,
metal/metalloid oxides, metal/metalloid nitrides, metal/metalloid
carbides, metal/metalloid sulfides, metal/metalloid silicates,
metal/metalloid phosphates or combinations thereof. As used herein,
inorganic particles include carbon particles, such as fullerenes,
carbon black, graphite and combinations thereof. Inorganic
particles excluding carbon particles can be referred to as
non-carbon inorganic particles. Metalloids are elements that
exhibit chemical properties intermediate between or inclusive of
metals and nonmetals. Metalloid elements include silicon, boron,
arsenic, antimony, and tellurium. While phosphorous is located in
the periodic table near the metal elements, it is not generally
considered a metalloid element. However, P.sub.2O.sub.5 and doped
forms of P.sub.2O.sub.5 are good optical materials similar to some
metalloid oxides, and other optical materials doped with
phosphorous, e.g., in the form of P.sub.2O.sub.5, can have
desirable optical properties. For convenience, as used herein
including in the claims, phosphorous is also considered a metalloid
element.
The inorganic particles can be incorporated at a range of loadings
into the blends. High inorganic particle loadings of up to about 50
weight percent or greater can be achieved with well dispersed
particles. In addition, in embodiments involving chemically bonded
composites, the amount of the linker compounds bonded to the
inorganic particles can be adjusted to vary the degree of
crosslinking obtained with the polymer.
In some aspects of the invention, polymer inorganic-particle blends
comprise polymer-inorganic particle composites with chemical
bonding between the inorganic particles and the polymer. In other
embodiments, the blends comprise mixtures of inorganic particles
and polymers. The composition of the components of the blends and
the relative amounts of the components can be selected to yield
desired properties, such as optical properties.
In embodiments of the polymer-inorganic particle blends involving
chemical bonding between the polymer and the inorganic particles,
the polymer is selected or modified to include appropriate
functional groups to chemically bond with the inorganic particles
or with functional groups of a linker compound. A linker compound
can facilitate the formation of the resulting composite.
Specifically, in these embodiments, the composites include a
monomer/polymer component, inorganic particles, and linker
compounds that bridge the inorganic particles and the
monomer/polymer. In the case of monomer units being joined to the
linker compound, a polymer is formed with the formation of the
composite. For simplicity in notation, the monomer/polymer unit
joined with the linker and assembled into the composite will be
referred to generally as a polymer, although it is recognized that
in some cases the unit can be a monomer or polymer, such as a
dimer, trimer or larger polymer structures. The molecular weights
of the polymers can be selected to vary the properties of the
resulting composite.
In some embodiments, it may be advantageous to use collections of
inorganic particles having an average diameter of less than about
500 nanometers (nm). Suitable nanoparticles can be formed, for
example, by flame synthesis, combustion, or sol gel approaches.
Methods for synthesizing inorganic particles with particular high
uniformity include radiation-based pyrolysis/laser pyrolysis in
which light from an intense radiation source drives the reaction to
form the particles. For convenience, this application refers to
radiation-based pyrolysis and laser pyrolysis interchangeably,
since a suitable intense source of electromagnetic radiation can be
used in place of a laser. Laser pyrolysis is useful in the
formation of particles that are highly uniform in composition,
crystallinity and size.
The use of nanoscale particles within the polymer/inorganic
particle blends can impart improved and/or desired properties for
some applications. In particular, for the formation of optical
materials, nanoparticles can provide desirable optical performance
due to desirable optical properties, such as generally decreased
scattering. High-quality nanoparticles are desirable for the
generation of homogeneously mixed nanoparticle-polymer blends with
well-defined optical properties. Specifically, it is desirable to
have particles in which the primary particles are not agglomerated
such that the primary particles can be dispersed effectively to
form the composite. High-quality nanoparticles to form
nanocomposites can be produced on a commercial scale, as described
in U.S. Pat. No. 5,958,348 to Bi et al., entitled "Efficient
Production of Particles By Chemical Reaction," incorporated herein
by reference, and as described further below.
Since a wide range of inorganic particles and polymers can be
incorporated into the composites described herein, the composites
are suitable for a wide range of applications. Specifically, the
materials and structures described herein are suitable for
applications including, for example, structural applications,
electronics applications and optical applications. Optical
applications are described herein in more detail, although all
applications are contemplated for the improved structures involving
the polymer-inorganic particle blends. One significant advantage
from the use of polymer-inorganic particle blends is the ability to
control physical properties such as photonic or electronic
parameters over a wide range. For example, if the inorganic
particles have a high index-of-refraction, a variety of optical
devices or optical coatings can be formed over wide range and
controllable values of index-of-refraction. High
index-of-refraction materials are desirable to control light
propagation. The index-of-refraction of the composite can be
controlled by adjusting particle loading.
The ability to control index-of-refraction has been demonstrated
through the study of the photonic/optical properties of
nanoparticle-polymer composites. For example, the refractive index,
which determines the propagation of light within a material or
device structure, can be studied as a function of parameters of the
blend. Optical observations from polymer-inorganic nanoparticle
composites are presented in the examples below. In particular,
structural and optical properties of individual nano-TiO.sub.2
particles are also described to show correlations between
properties of nanoparticles and subsequent nanocomposites. The
optical measurements with nano-TiO.sub.2-based polymer composites
confirm the ability to obtain high refractive index
polymer-inorganic particle blends.
Controlled management of the refractive index (n) is important for
photonic device applications. Index engineering as described herein
includes formation of materials with desired refractive indices and
their contrast with indices of adjacent materials at interfaces in
an optical structure. Modulation of the refractive index by
external fields may be achieved also.
Generally, the polymer-inorganic particle blends provide both for
considerable versatility with respect to composition and
corresponding properties as well as processing versatility for the
formation of structures ranging from the straightforward to the
complex. For optical applications, index-of-refraction can be
selected through the corresponding selection of components of the
blend and the particle loading. Conventional silica glass exhibits
n.about.1.45, depending on the dopant and the level of doping. At
the other end of the range, compound semiconductors such as InP
have n.about.3.4. There is a large gap in between the two regions,
which has not been covered adequately by existing materials
systems. Even if the gap is covered by multiple materials, there is
a significant challenge to form a workable interface between them.
In contrast, polymer-inorganic particle blends formed with
inorganic particles, such as nanoparticles, and a polymer matrix
provide the ability to select a particular desired value of
index-of-refraction. It has been found that once the high-index
particles, in particular nanoparticles, and a polymer host material
are chosen, the loading level of the nanoparticles directly
determines the index of the entire composite.
By combining a plurality of materials of which one or more is a
polymer-inorganic particle blend, interfaces can be formed between
materials within structures such that the overall properties and/or
functionality have desired features. For optical applications, the
optical interface can involve optical materials with a selected
difference in indices-of-refraction between the different
materials. The polymer-inorganic particle blends can be used to
engineer the index-of-refraction, which can be used to reduce the
size of optical components. In particular, high index contrast at
optical interfaces can be used to reduce device size, i.e.,
miniaturization.
In addition, the polymer-inorganic particle blends can be formed
into heterostructures designed for particular applications. For
some optical applications, the polymer-inorganic particle blends
can be formed into periodic structures. The formation of periodic
structures can be particularly advantageous in optical structures
for the formation of, for example, structures with periodically
modulated index-of-refraction. Optical materials with period
variation in index-of-refraction can be used to form gratings or
photonic crystals. The periodicity can extend in one, two or three
dimensions.
Using polymer-inorganic particle blends, high index blends can be
formed in association with low index materials, such as polymers
with no particle loadings. With these associated materials,
interfaces can be formed with large changes in index-of-refraction
between the two materials at the interface. This large change in
index-of-refraction can be used advantageously for the reflection
of light and/or the confinement of light within a material. In
particular, these large index changes can be used advantageously in
the formation of displays and other optical devices.
In particular, the use of polymer/inorganic particle composites is
particularly appropriate for the formation of devices with a
selected dielectric constant/index-of-refraction. Through
index-of-refraction engineering, the materials can be designed
specifically for a particular application through corresponding
selection of the index-of-refraction. Appropriate selection of
index-of-refraction can be important for the preparation of either
electrical or optical materials. The index-of-refraction is
approximately the square root of the dielectric constant when there
is no optical loss, so that the engineering of the
index-of-refraction corresponds to the engineering of the
dielectric constant. Thus, the index-of-refraction/dielectric
constant is related to both the optical and electrical response of
a particular material. Index-of-refraction engineering can be
especially advantageous in the design of optical or electrical
interconnects. The processing approaches described herein,
including for example the self-assembly approaches, can be used to
control domain size of materials forming devices and/or periodicity
of the material compositions/index-of-refraction. Structure
diameters and periodicities can be obtained on a submicron scale.
Desirable size and/or periodicity length scales generally depend on
the wavelengths of light. In addition, small size/periodicity
scales can be used if index-of-refraction values change by larger
amounts at interfaces. The use of nanoparticles and/or the ability
to form submicron scale structures provides the ability to form
quantum effect devices.
The polymer-inorganic particle blends can be processed using many
standard polymer-processing approaches. Particularly suitable
approaches generally depend on the specific structure being formed.
However, in the formation of interfaces between the
polymer-inorganic particle blends and other materials, certain
approaches can be particularly suitable. For example, uniform
layers can be applied by spin coating a solvated blend onto a
substrate, such as a silicon wafer. The layers can be stacked by
spin coating the materials sequentially. The solvents can be
selected such that solvent used during the application of one layer
does not dissolve a previously applied layer. In addition,
extrusion of a solvated blend or a melt can be used to form
interfaces. The multiple layers may or may not be coextruded.
Calendering can be used to improve the qualities of the interface.
Other molding and coating approaches can also be used. In general,
the processing of the polymer-inorganic particle blends may or may
not involve a substrate.
In addition, polymer-inorganic particle blends can be processed
using self-assembly techniques to form periodic structures. In
particular, for some optical applications, self-assembly techniques
can be used to form periodic optical structures with periodic
interfaces between two materials with a difference in value of
index-of-refraction between the two materials. Generally, the
periodic structure includes a polymer-inorganic particle blend as
one or both of the periodically varying materials in the periodic
structure in one dimension, two dimensions or three dimensions. The
two dimensional variation in index-of-refraction can be used to
construct two-dimensional photonic crystals. Similarly, a three
dimensional variation in index-of-refraction can be used to
construct three-dimensional photonic crystals. Three-dimensional
photonic crystals may be used to form an ideal solid state laser
without natural emission due to a photonic band gap. On the other
hand, two-dimensional photonic crystals may lead to integration of
surface emitting devices and waveguides to form
wavelength-division-multiplexers.
The polymer-inorganic particle blends can be advantageously
incorporated into a variety of devices, especially optical devices.
Relevant devices include, for example, optical attenuator, optical
splitter/coupler, optical switch, modulator, interconnect, optical
isolator, optical add-drop multiplexer (OADM), optical amplifier,
optical polarizer, optical circulator, phase shifter, optical
mirror/reflector, optical phase-retarder, optical detector,
displays, micro-electromechanical structures (MEMS), tunable
filters, optical switches, Bragg gratings, mirrors, band pass
filters, arrayed waveguide gratings (AWG), lasers, photonic
crystals and quasicrystals. The devices can be placed within
optical fibers or on planar optical structures. In particular,
within planar optical structures, the devices can be part of a
planar optical circuit with integrated optical devices.
Polymer-Inorganic Particle Blends
The particle-inorganic particle blends involve inorganic particles
distributed throughout a polymer matrix such that the resulting
blend incorporates aspects of both the inorganic particles and the
polymer. The inorganic particles may or may not be chemically
bonded to the polymer. The bonding of the inorganic particle to the
polymer can involve a linker that is used to activate the surface
of the inorganic particles for bonding with the polymer. Suitable
blends can involve either low particle loadings or high particle
loadings depending on the particular application. Similarly, the
composition of the polymer component and the inorganic particle
components can be selected to achieve desired properties of the
resulting blend. The blends, especially polymer-inorganic particle
composites, may represent a synergistic effect of the combined
component.
The inorganic particles can be incorporated at a range of loadings
into the composite. Composites with low particle loadings can be
produced with high uniformity. Low loadings, such as one or two
percent or less, can be desirable for some applications. In
addition, high inorganic particle loadings can be achieved with
well-dispersed particles. In addition, high inorganic particle
loadings of up to about 80 weight percent or greater can be
achieved with well dispersed particles. In general, the inorganic
particle loadings are from about 0.1 weight percent to about 90
weight percent, in other embodiments from about 1 weight percent to
about 85 weight percent, in further embodiments from about 3 weight
percent to about 80 weight percent, in additional embodiments from
about 5 weight percent to about 65 weight percent and in some
embodiments from about 10 to about 50 weight percent. A person of
skill in the art will recognize that other ranges within these
explicit ranges are contemplated and are within the present
disclosure. In addition, the amount the linker compounds bonded to
the inorganic particles can be adjusted to vary the degree of
crosslinking obtained with the polymer.
As noted above, the polymer-inorganic particle blends can involve
chemical bonding between the inorganic particles and the polymers.
For convenience, blends having chemical bonding between at least a
portion of the inorganic particles and the polymer are called
polymer-inorganic particle composites. Chemical bonding is
considered to broadly cover bonding with some covalent character
with or without partial ionic bonding character and can have
properties of ligand-metal bonding. In other embodiments, the
inorganic particles are simply embedded within the polymer matrix
by the physical properties of the matrix. For convenience, blends
not involving chemical bonding between the inorganic particles and
the polymer matrix are called polymer-inorganic particle mixtures.
Of course, polymer-inorganic particle mixtures generally involve
non-bonding electrostatic interactions, such as van der Waals
interactions, between the polymer and the inorganic particles.
While mixtures are suitable in many contexts, the formation of
polymer-inorganic particle composites can have advantages with
respect to stability and uniformity of the blend. Specifically,
high particle loadings can be achieved in a composite without
agglomeration of the particles, provided that the particles are
functionalized with groups that do not easily bond to themselves,
which can result in the formation of hard agglomerates. In
addition, in relevant embodiments, the amount the linker compounds
bonded to the inorganic particles can be adjusted to vary the
degree of crosslinking obtained with the polymer.
The composites with bonding between the polymer and the particles
comprise a monomer/polymer component, inorganic particles, and
linker compounds that bridge the inorganic particles and the
monomer/polymer. In the case of monomer units being joined to the
linker compound, a polymer is formed with the formation of the
composite. For simplicity in notation, the monomer/polymer unit
joined with the linker and assembled into the composite will be
referred to generally as a polymer, although it is recognized that
in some cases the unit can be a monomer or polymer, such as a
dimer, trimer or larger polymer structures.
The linker compounds have two or more functional groups. One
functional group of the linker is suitable for chemical bonding to
the inorganic particles. Chemical bonding is considered to broadly
cover bonding with some covalent character with or without polar
bonding and can have properties of ligand-metal bonding along with
various degrees of ionic bonding. The functional group is selected
based on the composition of the inorganic particle. Another
functional group of the linker is suitable for covalent bonding
with the polymer. Covalent bonding refers broadly to covalent bonds
with sigma bonds, pi bonds, other delocalized covalent bonds and/or
other covalent bonding types, and may be polarized bonds with or
without ionic bonding components and the like. Convenient linkers
include functionalized organic molecules.
Various structures can be formed based on the fundamental idea of
forming the chemically bonded polymer/inorganic particle
composites. The structures obtained will generally depend on the
relative amounts of polymer/monomers, linkers and inorganic
particles as well as the synthesis process itself. Linkers may be
identified also as coupling agents or crosslinkers. Furthermore, in
some embodiments, polymer-inorganic particle composites, as well as
polymer-inorganic particle blends, can comprise a plurality of
different polymers and/or a plurality of different inorganic
particles. Similarly, if a poly-inorganic particle blend comprises
a plurality of different polymer and/or a plurality of different
inorganic particles, all of the polymer and/or inorganic particles
can be chemically bonding within the composite or, alternatively,
only a fraction of the polymers and inorganic particles can be
chemically bonded within the composite. If only a fraction of the
polymer and/or inorganic particles are chemically bonded, the
fraction bonded can be a random portion or a specific fraction of
the total polymer and/or inorganic particles.
To form the desired composites, the inorganic particles can be
modified on their surface by chemical bonding to one or more linker
molecules. The ratio of linker composition to inorganic particles
can be at least one linker molecule per inorganic particle. The
linker molecules surface modify the inorganic particles, i.e.,
functionalize the inorganic particles. While the linker molecules
can bond to the inorganic particles, they can be, but are not
necessarily, bonded to the inorganic particles prior to bonding to
the polymers. They can be bonded first to the polymers and only
then bonded to the particles. Alternatively, they can bond to the
two species simultaneously.
In some embodiments, the linker is applied to form at least a
significant fraction of a monolayer on the surface of the
particles. In particular, for example, at least about 20% of a
monolayer can be applied to the particles, and in other
embodiments, at least about 40% of a monolayer can be applied.
Based on the measured BET surface areas of the particles, a
quantity of linker can be used corresponding up to coverage about
1/2, 1 and 2 of the particle surface relative to a monolayer of the
linker. A person of ordinary skill in the art will recognize that
other ranges within these explicit ranges are contemplated and are
within the present disclosure. A monolayer is calculated based on
measured surface area of the particles and an estimate of the
molecular radius of the linker based on accepted values of the
atomic radii. Excess linker reagent can be added because not all of
the linker binds and some self-polymerization of the linker reagent
can take place. To calculate the coverage, the linker can be
assumed to bond to the particle normal to the surface. This
calculation provides an estimate of the coverage. It has been found
experimentally that higher coverage could be placed over the
surface of the particles than estimated from these calculations.
With these high linker coverages, the linkers presumably form a
highly crosslinked structure with the polymers. At each inorganic
particle, multi-branched crosslinking structures are formed.
The inorganic particles can be bonded through the linker compound
into the polymer structure, or the particles can be grafted to
polymer side groups. The bonded inorganic particles can, in most
embodiments, crosslink the polymer. Specifically, most embodiments
involve star crosslinking of a single inorganic particle with
several polymer groups. The structure of the composite can
generally be controlled by the density of linkers, the length of
the linkers, the chemical reactivity of the coupling reaction, the
density of the reactive groups on the polymer as well as the
loading of particles and the molecular weight range of the polymer
(i.e., monomer/polymer units). In alternative embodiments, the
polymer has functional groups that bond directly with the inorganic
particles, either at terminal sites or at side groups. In these
alternative embodiments, the polymer includes functional groups
comparable to appropriate linker functional groups for bonding to
the inorganic particles.
A range of polymers is suitable for incorporation into the
composites, including, without limitation, organic polymers,
inorganic polymers, such as polysiloxanes, and combinations and
copolymers thereof. If the polymers are formed prior to reacting
with the functionalized inorganic particles, the molecular weights
of the polymers can be selected to vary to properties of the
resulting composite. The polymer is selected or synthesized to
include appropriate functional groups to covalently bond with
functional groups of the linker compound.
The frame of the linker supporting the functional groups is
generally an organic compound, although it may also include silyl
and/or siloxy moieties. The organic linker frame can comprise any
reasonable organic moiety including, for example, linear or
branched carbon chains, cyclical carbon moieties, saturated carbon
moieties, unsaturated carbon moieties, aromatic carbon units,
halogenated carbon groups and combinations thereof. The structure
of the linker can be selected to yield desirable properties of the
composite. For example, the size of the linker is a control
parameter that may affect the periodicity of the composite and the
self-organization properties.
Many different types of polymers are suitable as long as they have
terminal groups and/or preferably side groups capable of bonding to
a linker. Suitable organic polymers include, for example,
polyamides (nylons), polyimides, polycarbonates, polyurethanes,
polyacrylonitrile, polyacrylic acid, polyacrylates,
polyacrylamides, polyvinyl alcohol, polyvinyl chloride,
heterocyclic polymers, polyesters, modified polyolefins and
copolymers and mixtures thereof. Composites formed with nylon
polymers, i.e., polyamides, and inorganic nanoparticles can be
called Nanonylon.TM.. Suitable polymers include conjugated polymers
within the polymer backbone, such as polyacetylene, and aromatic
polymers within the polymer backbone, such as poly(p-phenylene),
poly(phenylene vinylene), polyaniline, polythiophene,
poly(phenylene sulfide), polypyrrole and copolymers and derivatives
thereof. Some polymers can be bonded to linkers at functional side
groups. The polymer can inherently include desired functional
groups, can be chemically modified to introduce desired functional
groups or copolymerized with monomer units to introduce portions of
desired functional groups. Similarly, some composites include only
a single polymer/monomer composition bonded into the composite.
Within a crosslinked structure, a polymer is identifiable by 3 or
more repeat units along a chain, except for hydrocarbon chains
which are not considered polymers unless they have a repeating side
group or at least about 50 carbons-carbon bonds within the
chain.
Preferred silicon-based polymers include polysilanes, polysiloxane
(silicone) polymers, such as poly(dimethylsiloxane) (PDMS) and
copolymers and mixtures thereof as well as copolymers and mixtures
with organic polymers. Polysiloxanes are particularly suitable for
forming composites with grafted inorganic particles. To form these
grafted composites, the polysiloxanes can be modified with amino
and/or carboxylic acid groups. Polysiloxanes are desirable polymers
because of their transparency to visible and ultraviolet light,
high thermal stability, resistance to oxidative degradation and its
hydrophobicity. Other inorganic polymers include, for example,
phosphazene polymers (phosphonitrile polymers).
Appropriate functional groups for binding with the polymer depend
on the functionality of the polymer. Generally, the functional
groups of the polymers and the linker can be selected appropriately
based on known bonding properties. For example, carboxylic acid
groups bond covalently to thiols, amines (primary amines and
secondary amines) and alcohol groups. As a particular example,
nylons can include unreacted carboxylic acid groups, amine groups
or derivatives thereof that are suitable form covalently bonding to
linkers. In addition, for bonding to acrylic polymers, a portion of
the polymer can be formed from acrylic acid or derivatives thereof
such that the carboxylic acid of the acrylic acid can bond with
amines (primary amines and secondary amines), alcohols or thiols of
a linker. The functional groups of the linker can provide selective
linkage either to only particles with particular compositions
and/or polymers with particular functional groups. Other suitable
functional groups for the linker include, for example, halogens,
silyl groups (--SiR.sub.3-xH.sub.x), isocyanate, cyanate,
thiocyanate, epoxy, vinyl silyls, silyl hydrides, silyl halogens,
mono-, di- and trihaloorganosilane, phosphonates, organometalic
carboxylates, vinyl groups, allyl groups and generally any
unsaturated carbon groups (--R'--C.dbd.C--R''), where R' and R''
are any groups that bond within this structure. Selective linkage
can be useful in forming composite structures that exhibit
self-organization.
Upon reaction of the polymer functional groups with the linker
functional groups, the identity of initial functional groups is
merged into a resultant or product functional group in the bonded
structure. A linkage is formed that extends from the polymer. The
linkage extending from the polymer can include, for example, an
organic moiety, a siloxy moiety, a sulfide moiety, a sulphonate
moiety, a phosphonate moiety, an amine moiety, a carbonyl moiety, a
hydroxyl moiety, or a combination thereof. The identity of the
original functional groups may or may not be apparent depending on
the resulting functional group. The resulting functional groups
generally can be, for example, an ester group, an amide group, an
acid anhydride group, an ether group, a sulfide group, a disulfide
group, an alkoxy group, a hydrocarbyl group, a urethane group, an
amine group, an organo silane group, a hydridosilane group, a
silane group, an oxysilane group, a phosphonate group, a sulphonate
group or a combination thereof.
If a linker compound is used, one resulting functional group
generally is formed where the polymer bonds to the linker and a
second resulting functional group is formed where the linker bonds
to the inorganic particle. At the inorganic particle, the
identification of the functional group may depend on whether
particular atoms are associated with the particle or with the
functional group. This is just a nomenclature issue, and a person
of skill in the art can identify the resulting structures without
concern about the particular allocation of atoms to the functional
group. For example, the bonding of a carboxylic acid with an
inorganic particle may result in a group involving bonding with a
non-metal/metalloid atom of the particle; however, an oxo group is
generally present in the resulting functional group regardless of
the composition of the particle. Ultimately, a bond extends to a
metal/metalloid atom.
Appropriate functional groups for bonding to the inorganic
particles depends on the character of the inorganic particles. U.S.
Pat. No. 5,494,949 to Kinkel et al., entitled "SURFACE-MODIFIED
OXIDE PARTICLES AND THEIR USE AS FILLERS AND MODIFYING AGENTS IN
POLYMER MATERIALS," incorporated herein by reference, describes the
use of silylating agents for bonding to metal/metalloid oxide
particles. The particles have alkoxy modified silane for bonding to
the particles. For example, preferred linkers for bonding to
metal/metalloid oxide particles include
R.sup.1R.sup.2R.sup.3--Si--R.sup.4, where R.sup.1, R.sup.2, R.sup.3
are alkoxy groups, which can hydrolyze and bond with the particles,
and R.sup.4 is a group suitable for bonding to the polymer.
Trichlorosilicate (--SiCl.sub.3) functional groups can react with
an hydroxyl group at the metal oxide particle surface by way of a
condensation reaction.
Generally, thiol groups can be used to bind to metal sulfide
particles and certain metal particles, such as gold, silver,
cadmium and zinc. Carboxyl groups can bind to other metal
particles, such as aluminum, titanium, zirconium, lanthanum and
actinium. Similarly, amines and hydroxide groups would be expected
to bind with metal oxide particles and metal nitride particles, as
well as to transition metal atoms, such as iron, cobalt, palladium
and platinum.
The identity of the linker functional group that bonds with the
inorganic particle may also be modified due to the character of the
bonding with the inorganic particle. One or more atoms of the
inorganic particle are involved in forming the bond between the
linker and the inorganic particle. It may be ambiguous if an atom
in the resulting linkage originates from the linker compound or the
inorganic particle. In any case, a resulting or product functional
group is formed joining the linker molecule and the inorganic
particle. The resulting functional group can be, for example, one
of the functional groups described above resulting from the bonding
of the linker to the polymer. The functional group at the inorganic
particle ultimately bonds to one or more metal/metalloid atoms.
In some embodiments, the polymer incorporates the inorganic
particles into the polymer network. This can be performed by
reacting a functional group of the linker compound with terminal
groups of a polymer molecule. Alternatively, the inorganic
particles can be present during the polymerization process such
that the functionalized inorganic particles are directly
incorporated into the polymer structure as it is formed. In other
embodiments, the inorganic particles are grafted onto the polymer
by reacting the linker functional groups with functional groups on
polymer side groups. In any of these embodiments, the surface
modified/functionalized inorganic particles can crosslink the
polymer if there are sufficient linker molecules, i.e., enough to
overcome energetic barriers and form at least two or more bonded
links to the polymer. Generally, an inorganic particle will have
many linkers associated with the particle. Thus, in practice, the
crosslinking depends on the polymer-particle arrangement,
statistical interaction of two crosslinking groups combined with
molecular dynamics and chemical kinetics.
In some embodiments, the composite is formed into localized
structures by self-assembly. The composition and/or structure of
the composite can be selected to encourage self-organization of the
composite itself. For example, block copolymers can be used such
that the different blocks of the polymer segregate, which is a
standard property of many block copolymers. Suitable block
copolymers include, for example, polystyrene-block-poly(methyl
methacrylate), polystyrene-block-polyacrylamide,
polysiloxane-block-polyacrylate and mixtures thereof. These block
copolymers can be modified to include appropriate functional groups
to bond with the linkers. For example, polyacrylates can be
hydrolyzed or partly hydrolyzed to form carboxylic acid groups, or
acrylic acid moieties can be substituted for all or part of the
acrylated during polymer formation if the acid groups do not
interfere with the polymerization. Alternatively, the ester groups
in the acrylates can be substituted with ester bonds to diols or
amide bonds with diamines such that one of the functional groups
remains for bonding with a linker. Block copolymers with other
numbers of blocks and other types of polymer compositions can be
used.
The inorganic particles can be associated with only one of the
polymer compositions within the block such that the inorganic
particles are segregated together with that polymer composition
within the segregation block copolymer. For example, an AB di-block
copolymer can include inorganic particles only within block A.
Segregation of the inorganic particles can have functional
advantages with respect to taking advantage of the properties of
the inorganic particles. Similarly, tethered inorganic particles
may separate relative to the polymer by analogy to different blocks
of a block copolymer if the inorganic particles and the
corresponding polymers have different solvation properties. In
addition, the nanoparticles themselves can segregate relative to
the polymer to form a self-organized structure.
Other ordered copolymers include, for example, graft copolymers,
comb copolymers, star-block copolymers, dendrimers, mixtures
thereof and the like. Ordered copolymers of all types can be
considered a polymer blend in which the polymer constituents are
chemically bonded to each other. Physical polymer blends may also
be used and may also exhibit self-organization, as described in the
examples below. Polymer blends involve mixtures of chemically
distinct polymers. The inorganic particles may bond to only a
subset of the polymer species, as described above for block
copolymers. Physical polymer blends can exhibit self-organization
similar to block copolymers. The presence of the inorganic
particles can sufficiently modify the properties of the composite
that the interaction of the polymer with inorganic particles
interacts physically with the other polymer species differently
than the native polymer alone. In particular, the presence of
nanoparticles within the polymer-inorganic particle blends can
result in a blend that is sensitive to weak fields due to the small
particle size. This sensitivity can be advantageously used in the
formation of devices. Processes making use of small particles
generally can be referred to as a soft matter approach.
Regardless of the self-organization mechanism, some self-organized
composites involve nanoparticles aligned with periodicity in a
superstructure or super crystal structure, i.e., a periodic array
of crystalline particles. The particles may or may not be
crystalline themselves yet they will exhibit properties due to the
ordered structure of the particles. Photonic crystals make use of
these crystal superstructures, as described further below.
Exemplary embodiments of polymer-inorganic particle composites are
described further in and commonly assigned U.S. patent application
Ser. No. 09/818,141, now U.S. Pat. No. 6,599,631 to Kambe et al.,
entitled "Polymer-Inorganic Particle Composites," incorporated
herein by reference.
Inorganic Particles
In general, any reasonable inorganic particles can be used to form
the blends. In some embodiments, the particles have an average
diameter of no more than about one micron. For some applications of
interest, the composition of the particles is selected to impart
desired properties to the composite. Thus, in the formation of
optical materials for example, the optical properties of both the
polymer and the inorganic particles can be significant. It is
expected that the index-of-refraction of the composite material is
roughly the linear combination based on the weight ratios of the
index-of-refractions of the inorganic particles and the polymer to
quite high particle loadings by weight.
Suitable nanoparticles can be formed, for example, by laser
pyrolysis, flame synthesis, combustion, or sol gel approaches. In
particular, laser pyrolysis is useful in the formation of particles
that are highly uniform in composition, crystallinity and size.
Laser pyrolysis involves light from an intense light source that
drives the reaction to form the particles. Laser pyrolysis is an
excellent approach for efficiently producing a wide range of
nanoscale particles with a selected composition and a narrow
distribution of average particle diameters. Alternatively,
submicron particles can be produced using a flame production
apparatus such as the apparatus described in U.S. Pat. No.
5,447,708 to Helble et al., entitled "Apparatus for Producing
Nanoscale Ceramic Particles," incorporated herein by reference.
Furthermore, submicron particles can be produced with a thermal
reaction chamber such as the apparatus described in U.S. Pat. No.
4,842,832 to Inoue et al., "Ultrafine Spherical Particles of Metal
Oxide and a Method for the Production Thereof," incorporated herein
by reference. In addition, various solution-based approaches can be
used to produce submicron particles, such as sol gel
techniques.
Highly uniform particles can be formed by radiation based
pyrolysis, e.g., laser pyrolysis, which can be used to form
submicron particles with extremely uniform properties with a
variety of selectable compositions. For convenience, radiation
based pyrolysis is referred to as laser pyrolysis since this
terminology reflects the convenience of lasers as a radiation
source. Laser pyrolysis approaches discussed herein incorporate a
reactant flow that can involve vapors, aerosols or combinations
thereof to introduce desired elements into the flow stream. The
versatility of generating a reactant stream with vapor and/or
aerosol precursors provides for the generation of particles with a
wide range of potential compositions.
Small particles can provide processing advantages with respect to
forming small structures and smooth surfaces. In addition, small
particles have desirable properties for optical applications
including, for example, a shifted absorption spectrum and reduced
scattering, which results in lower scattering loss. Thus, small
particle exhibit observable quantum effects due to their small
size, which can affect the optical properties of corresponding
polymer-inorganic particle blends.
A collection of submicron/nanoscale particles may have an average
diameter for the primary particles of less than about 500 nm,
preferably from about 2 nm to about 100 nm, alternatively from
about 2 nm to about 75 nm, or from about 2 nm to about 50 nm. A
person of ordinary skill in the art will recognize that other
ranges within these specific ranges are covered by the disclosure
herein. Particle diameters are evaluated by transmission electron
microscopy.
The primary particles can have a roughly spherical gross
appearance, or they can have rod shapes, plate shapes or other
non-spherical shapes. Upon closer examination, crystalline
particles generally have facets corresponding to the underlying
crystal lattice. Amorphous particles generally have a spherical
aspect. Diameter measurements on particles with asymmetries are
based on an average of length measurements along the principle axes
of the particle.
Because of their small size, the primary particles tend to form
loose agglomerates due to van der Waals and other electromagnetic
forces between nearby particles. These agglomerates can be
dispersed in a dispersant to a significant degree based on the
primary particles, and in some embodiments essentially completely
to form dispersed primary particles. The size of the dispersed
particles can be referred to as the secondary particle size. The
primary particle size, of course, is the lower limit of the
secondary particle size for a particular collection of particles,
so that the average secondary particle size preferably is
approximately the average primary particle size. The secondary or
agglomerated particle size may depend on the subsequent processing
of the particles following their initial formation and the
composition and structure of the particles. In some embodiments,
the secondary particles have an average diameter no more than about
1000 nm, in additional embodiments no more than about 500 nm, in
further embodiments from about 2 nm to about 300 nm, in other
embodiments about 2 nm to about 100 nm, and alternatively about 2
nm to about 50 nm. A person of ordinary skill in the art will
recognize that other ranges within these specific ranges are
contemplated and are within the present disclosure. Secondary
particles sizes within a liquid dispersion can be measured by
established approaches, such as dynamic light scattering. Suitable
particle size analyzers include, for example, a Microtrac UPA
instrument from Honeywell based on dynamic light scattering, a
Horiba Particle Size Analyzer from Horiba, Japan and ZetaSizer
Series of instruments from Malvern based on Photon Correlation
Spectroscopy. The principles of dynamic light scattering for
particle size measurements in liquids are well established.
Even though the particles form loose agglomerates, the nanometer
scale of the primary particles is clearly observable in
transmission electron micrographs of the particles. The particles
generally have a surface area corresponding to particles on a
nanometer scale as observed in the micrographs. Furthermore, the
particles can manifest unique properties due to their small size
and large surface area per weight of material. For example, the
absorption spectrum of crystalline, nanoscale TiO.sub.2 particles
is shifted, as described in the examples below.
The primary particles can have a high degree of uniformity in size.
Laser pyrolysis generally results in particles having a very narrow
range of particle diameters. Furthermore, heat processing under
suitably mild conditions does not alter the very narrow range of
particle diameters. With aerosol delivery of reactants for laser
pyrolysis, the distribution of particle diameters is particularly
sensitive to the reaction conditions. Nevertheless, if the reaction
conditions are properly controlled, a very narrow distribution of
particle diameters can be obtained with an aerosol delivery system.
As determined from examination of transmission electron
micrographs, the primary particles generally have a distribution in
sizes such that at least about 95 percent, and preferably 99
percent, of the primary particles have a diameter greater than
about 40 percent of the average diameter and less than about 160
percent of the average diameter. Preferably, the primary particles
have a distribution of diameters such that at least about 95
percent, and preferably 99 percent, of the primary particles have a
diameter greater than about 60 percent of the average diameter and
less than about 140 percent of the average diameter. A person of
ordinary skill in the art will recognize that other ranges within
these specific ranges are covered by the disclosure herein.
Furthermore, in preferred embodiments no primary particles have an
average diameter greater than about 4 times the average diameter
and preferably 3 times the average diameter, and more preferably 2
times the average diameter. In other words, the particle size
distribution effectively does not have a tail indicative of a small
number of particles with significantly larger sizes. This is a
result of the small reaction region and corresponding rapid quench
of the particles. An effective cut off in the tail of the size
distribution indicates that there are less than about 1 particle in
10.sup.6 have a diameter greater than a specified cut off value
above the average diameter. High particle uniformity can be
exploited in a variety of applications. In particular, high
particle uniformity can lead to well controlled optical
properties.
In addition, the nanoparticles for incorporation into the blends
may have a very high purity level. Furthermore, crystalline
nanoparticles, such as those produced by laser pyrolysis, can have
a high degree of crystallinity. Similarly, the crystalline
nanoparticles produced by laser pyrolysis can be subsequently heat
processed to improve and/or modify the degree of crystallinity
and/or the particular crystal structure. Impurities on the surface
of the particles may be removed by heating the particles to achieve
not only high crystalline purity but high purity overall.
A basic feature of successful application of laser pyrolysis for
the production of desirable inorganic nanoparticles is the
generation of a reactant stream containing one or more
metal/metalloid precursor compounds, a radiation absorber and, in
some embodiments, a secondary reactant. The secondary reactant can
be a source of non-metal/metalloid atoms, such as oxygen, required
for the desired product and/or can be an oxidizing or reducing
agent to drive a desired product formation. A secondary reactant is
not needed if the precursor decomposes to the desired product under
intense light radiation. Similarly, a separate radiation absorber
is not needed if the metal/metalloid precursor and/or the secondary
reactant absorb the appropriate light radiation. The reaction of
the reactant stream is driven by an intense radiation beam, such as
a light beam, e.g., a laser beam. As the reactant stream leaves the
radiation beam, the particles are rapidly quenched.
A laser pyrolysis apparatus suitable for the production of
commercial quantities of particles by laser pyrolysis has been
developed using a reactant inlet that is significantly elongated in
a direction along the path of the laser beam. This high capacity
laser pyrolysis apparatus, e.g., 1 kilogram or more per hour, is
described in U.S. Pat. No. 5,958,348, entitled "Efficient
Production Of Particles By Chemical Reaction," incorporated herein
by reference. Approaches for the delivery of aerosol precursors for
commercial production of particles by laser pyrolysis is described
in copending and commonly assigned U.S. Pat. No. 6,193,936 to
Gardner et al., entitled "Reactant Delivery Apparatus,"
incorporated herein by reference.
In general, nanoparticles produced by laser pyrolysis can be
subjected to additional processing to alter the nature of the
particles, such as the composition and/or the crystallinity. For
example, the nanoparticles can be subjected to heat processing in a
gas atmosphere prior to use. Under suitably mild conditions, heat
processing is effective to modify the characteristics of the
particles without destroying the nanoscale size or the narrow
particle size distribution of the initial particles. For example,
heat processing of submicron vanadium oxide particles is described
in U.S. Pat. No. 5,989,514 to Bi et al., entitled "Processing Of
Vanadium Oxide Particles With Heat," incorporated herein by
reference.
A wide range of simple and complex submicron and/or nanoscale
particles have been produced by laser pyrolysis with or without
additional heat processing. In embodiments of particular interest
for the formation of polymer-inorganic particle blends, the
inorganic particles generally include metal or metalloid elements
in their elemental form or in compounds. Specifically, the
inorganic particles can include, for example, elemental metal or
elemental metalloid, i.e. un-ionized elements such as silver and
silicon, metal/metalloid oxides, metal/metalloid nitrides,
metal/metalloid carbides, metal/metalloid sulfides or combinations
thereof. In addition, there is the capability for producing
nano-particulate carbon materials. Complex systems of ternary and
quaternary compounds have also been made. In addition, uniformity
of these high quality materials is substantial. These particles
generally have a very narrow particle size distribution, as
described above. Availability of multiple types of nanoparticles
provides a significant increase in potential combinations between
nanoparticles and polymers.
With respect to the electrical properties of the particles, some
particles include compositions such that the particles are
electrical conducting, electrical insulators or electrical
semiconductors. Suitable electrical conductors include, for
example, elemental metals and some metal compositions. Electrical
conductors, such as metals, generally have a room temperature
resistivity of no more than about 1.times.10.sup.-3 Ohm-cm.
Electrical insulators generally have a room temperature resistivity
of at least about 1.times.10.sup.5 Ohm-cm. Electrical
semiconductors include, for example, silicon, CdS and in P.
Semiconducting crystals can be classified to include so called,
II-VI compounds, III-V compounds and group IV compounds, where the
number refers to the group in the periodic table. Semiconductors
are characterized by a large increase in conductivity with
temperature in pure form and an increase in electrical conductivity
by orders of magnitude upon doping with electrically active
impurities. Semiconductors generally have a band gap that results
in the observed conductivity behavior. At room temperature, the
conductivity of a semiconductor is generally between that of a
metal and a good electrical insulator.
Several different types of nanoscale particles have been produced
by laser pyrolysis. As used herein, inorganic particles include
carbon particles as carbonaceous solids, such as fullerenes,
graphite, and carbon black. Such nanoscale particles for light
reactive deposition can generally be characterized as comprising a
composition with a number of different elements that are present in
varying relative proportions, where the number and the relative
proportions are selected based on the application for the nanoscale
particles. Materials that have been produced (possibly with
additional processing, such as a heat treatment) or have been
described in detail for production by laser pyrolysis include, for
example, carbon particles, silicon, amorphous SiO.sub.2, doped
SiO.sub.2, crystalline silicon dioxide, titanium oxide (anatase and
rutile TiO.sub.2), MnO, Mn.sub.2O.sub.3, Mn.sub.3O.sub.4,
Mn.sub.5O.sub.8, vanadium oxide, silver vanadium oxide, lithium
manganese oxide, aluminum oxide (.gamma.-Al.sub.2O.sub.3,
delta-Al.sub.2O.sub.3 and theta-Al.sub.2O.sub.3), doped-crystalline
and amorphous alumina, tin oxide, zinc oxide, rare earth metal
oxide particles, rare earth doped metal/metalloid oxide particles,
rare earth metal/metalloid sulfides, rare earth doped
metal/metalloid sulfides, silver metal, iron, iron oxide, iron
carbide, iron sulfide (Fe.sub.1-xS), cerium oxide, zirconium oxide,
barium titanate (BaTiO.sub.3), aluminum silicate, aluminum
titanate, silicon carbide, silicon nitride, and metal/metalloid
compounds with complex anions, for example, phosphates, silicates
and sulfates. In particular, many materials suitable for the
production of optical materials can be produced by laser pyrolysis.
The production of particles by laser pyrolysis and corresponding
deposition as a coating having ranges of compositions is described
further in and commonly assigned U.S. patent application Ser. No.
10/027,906, now U.S. Pat. No. 6,952,504 to Bi et al., entitled
"Three Dimensional Engineering of Optical Structures," incorporated
herein by reference.
Submicron and nanoscale particles can be produced with selected
dopants using laser pyrolysis and other flowing reactor systems.
Amorphous powders and crystalline powders can be formed with
complex compositions comprising a plurality of selected dopants.
The powders can be used to form optical materials and the like.
Amorphous submicron and nanoscale powders and glass layers with
dopants, such as rare earth dopants and/or other metal dopants, are
described further in copending and commonly assigned U.S.
Provisional Patent Application Ser. No. 60/313,588 to Home et al.,
entitled "Doped Glass Materials," incorporated herein by reference.
Crystalline submicron and nanoscale particles with dopants, such as
rare earth dopants, are described further in and commonly assigned
U.S. patent application Ser. No. 09/843,195, now U.S. Pat. No.
6,692,660 to Kumar et al., entitled "High Luminescence Phosphor
Particles," incorporated herein by reference.
The dopants can be introduced at desired quantities by varying the
composition of the reactant stream. The dopants are introduced into
an appropriate host material by appropriately selecting the
composition in the reactant stream and the processing conditions.
Thus, submicron particles incorporating one or more metal or
metalloid elements as host composition with selected dopants,
including, for example, rare earth dopants and/or complex blends of
dopant compositions, can be formed. For embodiments in which the
host materials are oxides, an oxygen source should also be present
in the reactant stream. For these embodiments, the conditions in
the reactor should be sufficiently oxidizing to produce the oxide
materials.
Furthermore, dopants can be introduced to vary properties of the
resulting particles. For example, dopants can be introduced to
change the index-of-refraction of the particles that are
subsequently incorporated into the polymer-inorganic particle
blend. For optical applications, the index-of-refraction can be
varied to form specific optical devices that operate with light of
a selected frequency range. Dopants can also be introduced to alter
the processing properties of the material. Furthermore, dopants can
also interact within the materials. For example, some dopants are
introduced to increase the solubility of other dopants.
In some embodiments, the one or plurality of dopants are rare earth
metals or rare earth metals with one or more other dopant elements.
Rare earth metals comprise the transition metals of the group IIIb
of the periodic table. Specifically, the rare earth elements
comprise Sc, Y and the Lanthamide series. Other suitable dopants
comprise elements of the actinide series. For optical glasses, the
rare earth metals of particular interest as dopants comprise, for
example, Ho, Eu, Ce, Tb, Dy, Er, Yb, Nd, La, Y, Pr and Tm.
Generally, the rare earth ions of interest have a +3 ionization
state, although Eu.sup.+2 and Ce.sup.+4 are also of interest. Rare
earth dopants can influence the optical absorption properties that
can alter the application of the materials for the production of
optical amplifiers and other optical devices. Suitable non-rare
earth metal dopants for optical glasses comprise, for example, Bi,
Sb, Zr, Pb, Li, Na, K, Ba, B, Ge, W, Ca, Cr, Ga, Al, Mg, Sr, Zn,
Ti, Ta, Nb, Mo, Th, Cd and Sn.
In addition, suitable metal oxide dopants for aluminum oxide for
optical glass formation comprise cesium oxide (Cs.sub.2O), rubidium
oxide (Rb.sub.2O), thallium oxide (Tl.sub.2O), lithium oxide
(Li.sub.2O), sodium oxide (Na.sub.2O), potassium oxide (K.sub.2O),
beryllium oxide (BeO), magnesium oxide (MgO), calcium oxide (CaO),
strontium oxide (SrO) and barium oxide (BaO). Aluminum oxide
dopants can affect, for example, the index-of-refraction,
consolidation temperature and/or the porosity of the glass.
Suitable metal oxide dopants for infrared emitters comprise, for
example, cobalt oxide (CO.sub.3O.sub.4), Er.sub.2O.sub.3,
CrO.sub.2, Tm.sub.2O.sub.3, Nd.sub.2O.sub.3, Yb.sub.2O.sub.3,
Pr.sub.2O.sub.3, Dy.sub.2O.sub.3, and Ho.sub.2O.sub.3.
As noted above, laser pyrolysis has been used to produce a range of
powder compositions. The compositions can include multiple
metal/metalloid elements. A representative sample of references
relating to some of these powder materials are presented.
As a first example of nanoparticle production, the production of
silicon oxide nanoparticles is described in and commonly assigned
U.S. patent application Ser. No. 09/085,514, now U.S. Pat. No.
6,726,990 to Kumar et al., entitled "Silicon Oxide Particles,"
incorporated herein by reference. This patent application describes
the production of amorphous SiO.sub.2. The synthesis by laser
pyrolysis of silicon carbide and silicon nitride is described in
copending and commonly assigned U.S. patent application Ser. No.
09/433,202 to Reitz et al. filed on Nov. 5, 1999, entitled
"Particle Dispersions," incorporated herein by reference. The
production of silicon particles by laser pyrolysis is described in
an article by Cannon et al., J. of the American Ceramic Society,
Vol. 65, No. 7, pp. 330-335 (1982), entitled Sinterable Ceramic
Particles From Laser-Driven Reactions: II, Powder Characteristics
And Process Variables," incorporated herein by reference.
The production of titanium oxide nanoparticles and crystalline
silicon dioxide nanoparticles is described in and commonly
assigned, U.S. patent application Ser. No. 09/123,255, now U.S.
Pat. No. 6,387,531 to Bi et al., entitled "Metal (Silicon)
Oxide/Carbon Composites," incorporated herein by reference. In
particular, this application describes the production of anatase
and rutile TiO.sub.2. The production of aluminum oxide
nanoparticles is described in copending and commonly assigned, U.S.
patent application Ser. No. 09/136,483 to Kumar et al., entitled
"Aluminum Oxide Particles," incorporated herein by reference. In
particular, this application disclosed the production of
.gamma.-Al.sub.2O.sub.3. Suitable liquid, aluminum precursors with
sufficient vapor pressure of gaseous delivery include, for example,
aluminum s-butoxide (Al(OC.sub.4H.sub.9).sub.3). Also, a number of
suitable solid, aluminum precursor compounds are available
including, for example, aluminum chloride (AlCl.sub.3), aluminum
ethoxide (Al(OC.sub.2H.sub.5).sub.3), and aluminum isopropoxide
(Al[OCH(CH.sub.3).sub.2].sub.3).
Furthermore, mixed metal oxide nanoparticles have been produced by
laser pyrolysis along with or without subsequent heat processing,
as described in and commonly assigned U.S. patent application Ser.
No. 09/188,768, now U.S. Pat. No. 6,607,706 to Kumar et al.,
entitled "Composite Metal Oxide Particles," and Ser. No.
09/334,203, now U.S. Pat. No. 6,482,374 to Kumar et al., entitled
"Reaction Methods for Producing Ternary Particles," and U.S. Pat.
No. 6,136,287 to Home et al., entitled "Lithium Manganese Oxides
and Batteries," all three of which are incorporated herein by
reference. The formation of submicron and nanoscale particles
comprising metal/metalloid compounds with complex anions is
described in copending and commonly assigned U.S. patent
application Ser. No. 09/845,985 to Chaloner-Gill et al., entitled
"Phosphate Powder Compositions And Methods For Forming Particles
With Complex Anions," incorporated herein by reference. Suitable
complex anions include, for example, phosphates, silicates and
sulfates.
Formation of Polymer-Inorganic Particle Blends
Formation of the blends involves distributing the particles within
the polymer material such that the resulting blend forms a single
material. The polymerization process can be performed before
combining the particles with the polymer materials or in the
presence of the inorganic particles or some combination thereof.
The process for forming a particular blend generally depends on
whether the particles are simply dispersed within a polymer matrix
binder as a mixture or whether at least some of the particles are
covalently bonded to the polymer as a composite. The process for
forming the blend may involve dispersing the inorganic particles,
especially for the formation of composites. If a composite is
formed a linker molecule may be used to join the polymer and the
inorganic particle. The order for bonding the linker, the inorganic
particles and the polymer can be selected to yield a convenient
process.
The formation of a particle dispersion provides for the separation
of the particles such that the particles can be well dispersed
within the resulting blend. The use of a dispersion can result in a
more uniform blend with the particles approximately uniformly
distributed through the blend. The solvent, pH, ionic strength and
additives can be selected to improve the dispersion of the
particles. Greater dispersion of the particles and stability of the
dispersions helps to reduce agglomeration of the particles in the
resulting blend.
However, in alternative embodiments, the powders can be ground or
otherwise directly mixed with the polymer to disperse the particles
through the polymer. Mixing can be performed with or without the
presence of a solvent/dispersant. Commercial mixers or grinders,
for example, can be used to form the particle-polymer mixtures.
Heat, pressure and/or solvent/dispersant removal can be used to
bind particles within a polymer mixture in which the polymer
functions as a binder. Although at high particle loadings in a
mixture, the particles may be highly aggregated, unless possibly if
the particles were well dispersed prior to and during formation of
the mixture.
In some embodiments, the formation of a particle dispersion can be
a distinct step of the process. Preferably, a collection of
particles, e.g., nanoparticles, is well dispersed for uniform
introduction into a polymer blend, e.g., a composite. A liquid
phase particle dispersion can provide a source of small secondary
particles that can be used in the formation of desirable blend
structures. Desirable qualities of a liquid dispersion of inorganic
particles generally depend on the concentration of particles, the
composition of the dispersion and the formation of the dispersion.
Specifically, the degree of dispersion intrinsically depends on the
interparticle interactions, the interactions of the particles with
the liquid and the surface chemistry of the particles. Suitable
dispersants include, for example, water, organic solvents, such as
alcohols and hydrocarbons, and combinations thereof. The selection
of appropriate dispersants/solvents generally depends on the
properties of the particles. The degree of dispersion and stability
of the dispersion can be significant features for the production of
uniform composites without large effects from significantly
agglomerated particles.
Generally, the liquid dispersions refer to dispersions having
particle concentrations of no more than about 80 weight percent.
For the formation of a particle dispersion, the particular particle
concentration depends on the selected application. At
concentrations greater than about 50 weight percent, different
factors can be significant with respect to the formation and
characterization of the resulting viscous blend relative to
parameters that characterize the more dilute particle dispersions.
The concentration of particles affects the viscosity and can affect
the efficacy of the dispersion process. In particular, high
particle concentrations can increase the viscosity and make it more
difficult to disperse the particles to achieve small secondary
particle sizes, although the application of shear can assist with
particle dispersion.
Since many polymers are soluble in organic solvents, many
embodiments involve the formation of non-aqueous dispersions. In
addition, water based dispersions can include additional
compositions, such as surfactants, buffers and salts. For
particular particles, the properties of the dispersion can be
adjusted by varying the pH and/or the ionic strength. Ionic
strength can be varied by addition of inert salts, such as sodium
chloride, potassium chloride or the like. The presence of the
linker can effect the properties and stability of the dispersion.
The pH generally affects the surface charge of the dispersed
particles. The liquid may apply physical/chemical forces in the
form of solvation-type interactions to the particles that may
assist in the dispersion of the particles. Solvation-type
interactions can be energetic and/or entropic in nature.
The qualities of the dispersion generally depend on the process for
the formation of the dispersion. In dispersions, besides
chemical/physical forces applied by the dispersant and other
compounds in the dispersion, mechanical forces can be used to
separate the primary particles, which are held together by van der
Waals forces and other short range electromagnetic forces between
adjacent particles. In particular, the intensity and duration of
mechanical forces applied to the dispersion significantly
influences the properties of the dispersion. Mechanical forces can
be applied to the powders prior to dispersion in a solvent.
Alternatively, mechanical forces, such as shear stress, can be
applied as mixing, agitation, jet stream collision and/or
sonication following the combination of a powder or powders and a
liquid or liquids. Smaller secondary particles sizes are obtained
if there is more disruption of the agglomerating forces between the
primary particles.
The presence of small secondary particle sizes, e.g., close to the
primary particle size, can result in significant advantages in the
application of the dispersions for the formation of blends with
uniform properties. For example, smaller secondary particle sizes,
and generally small primary particle sizes, may assist with the
formation of smoother and/or smaller and more uniform structures
using the blends. In the formation of coatings, thinner and
smoother coatings can be formed with blends formed with inorganic
particle dispersions having smaller secondary particles.
Once the dispersion is formed, the dispersion may eventually
separate such that the particles collect on the bottom of the
container without continued mechanical stirring or agitation.
Stable dispersions have particles that do not separate out of the
dispersion. Different dispersions have different degrees of
stability. The stability of a dispersion depends on the properties
of the particles, the other compositions in the dispersion, the
processing used to form the dispersion and the presence of
stabilizing agents. Suitable stabilizing agents include, for
example, surfactants. In some embodiments, dispersions are
reasonably stable, such that the dispersions can be used without
significant separation during the subsequent processing steps
forming the blends, although suitable processing to form a blend
can be used involving constant mixing or the like to prevent
separation of the particle dispersion.
For the formation of composites, during formation or after
formation of the particle dispersion, the dispersion is interacted
with the linker molecules and/or the polymer. To form the desired
composites, the inorganic particles may be modified on their
surface by chemical bonding to one or more linker molecules.
Generally, for embodiments involving a linker, the linker is
soluble in the liquid used to form the inorganic particle
dispersion and/or the polymer dispersion so that the linker is
substantially homogeneously dissolved when bonding from solution.
Conditions for the combined particle dispersion and polymer
dispersion/solution can be suitable for the formation of bonds
between the linker, the inorganic particles and the polymer. The
order for adding the linker to the inorganic particles and the
polymer can be selected to achieve the desired processing
effectiveness. Once sufficient time has passed to complete the
bonding between the components of the composite, the composite can
be processed further.
The ratio of linker composition to inorganic particles preferably
is at least one linker molecular per inorganic particle. The linker
molecules surface modify the inorganic particles, i.e.,
functionalize the inorganic particles. While the linker molecules
bond to the inorganic particles, they are not necessarily bonded to
the inorganic particles prior to bonding to the polymers. They can
be bonded first to the polymers and only then bonded to the
particles. Alternatively, the components can be blended such that
bonding between the linker and the two species occurs
simultaneously.
The linker compound and the polymer/monomer components can be added
to the liquid with the particle dispersion simultaneously or
sequentially. The order of combining the various constituents can
be selected to achieve the desired results. The conditions within
the liquid preferably are suitable for the bond formation with the
linker and possibly other bond formation involving the
polymer/monomer constituents. Once the composite is formed, the
liquid can be removed or solidified to leave behind a structure
formed from the composite.
The polymer/monomer composition can be formed into a
solution/dispersion prior to addition to the inorganic particle
dispersion, or the polymer/monomer can be added as a solid to the
particle dispersion. In preferred embodiments, the polymer/monomer
compositions are soluble in the liquid used to form the particle
dispersion. If the polymer/monomer is not soluble/dispersible in
the particle dispersion, either the polymer/monomer solution or the
particle dispersion is slowly added to the other while mixing to
effect the reaction. Whether or not the polymer/monomer is first
solubilized separate from the inorganic particle dispersion may
depend on the kinetics of the polymer/monomer solubilization and on
the desired concentrations of the various solutions/dispersions.
Similarly, bonding kinetics can influence the order and details of
the mixing procedures.
In some embodiments, the reaction conditions and/or the presence of
a catalyst or the like is needed to initiate the reaction of the
linker with the inorganic particle and/or the polymer/monomer. In
these embodiments, the components can be mixed prior to the
adjustment of the reaction conditions of the addition of a
catalyst. Thus, a well mixed solution/dispersion can be formed
prior to the adjustment of the reaction conditions or addition of
the catalyst to form a more uniform composite.
Structures Incorporating Polymer-Inorganic Particle Blends
While the blends can be formed into free standing structures,
structures of interest can involving interfaces between a
polymer-inorganic particle blend and another material. The other
material at the interface may or may not itself be a
polymer-inorganic particle blend. The interface can be along a
planar surface, along an edge of an extended material and/or along
other types of surface either simple or complex. In some
embodiments of interest, the polymer-inorganic particle blend is an
optical material. In these embodiments, the other material may also
be an optical material such that the interface is an optical
interface. The interfaces can be incorporated into particular
structures to form devices of interest.
Referring to FIG. 1, structure 100 includes a first layer 102 of
polymer-inorganic particle blend and a second layer 104 of a second
material. First layer 102 contacts second layer 104 at interface
106. In this embodiment, interface 106 is planar, although other
non-planar interfaces of simple or complex geometry can be formed.
Additional layers can be formed from polymer-inorganic particle
blends and/or other materials. Specifically, the structure can
include three or more layers with adjacent layers having the same
or different compositions. If adjacent layers both are
polymer-inorganic particle blends, the layers can differ with
respect to the polymer, the inorganic particles and/or particle
loadings. In particular, adjacent layers can have different
particle loadings to adjust the differences in index-of-refraction
between the adjacent materials. The optical properties within a
layer depend on the index-of-refraction as well as the dimensions
including, for example, thickness. Planar structures, such as shown
in FIG. 1, have lengths in two dimensions that are at least an
order of magnitude, i.e., a factor of 10, larger than a thickness
along a dimension perpendicular to the two extended dimensions.
Referring to FIG. 2, structure 112 has a first material 114
comprising a polymer-inorganic particle blend and a second material
116. First material 114 and second material 116 form an interface
118 along an edge. An edge has at least one dimension that is at
least an order of magnitude smaller than an extended length
dimension of the structure. A more complex structure is depicted in
FIG. 3. Structure 124 includes a first material 126 comprising a
polymer-inorganic particle blend, a second material 128 and a third
material 130. First material 126, second material 128 and third
material 130 contact each other at interfaces 132, 134, 136, 138.
Various other structures involving polymer-inorganic particle
blends can be formed including, for example, more complex
structures with corresponding complex interfaces between adjacent
materials and/or structures with a network of interfaces that may
form optical pathways through the structure.
Generally, one of the materials within the structures of interest
comprises a polymer-inorganic particle blend. Suitable relative
proportions and compositions of the components of the blend are
described in detail above. Specific compositions can be selected
based on the desired properties, such as index-of-refraction, of
the material within the structure. The polymer-inorganic particle
blend can be a mixture or a composite. Polymer-inorganic particle
composites generally are more stable and have more dispersed
inorganic particles compared with mixtures, assuming appropriate
processes are used to form the composites. In addition, a
polymer-inorganic particle blend material can further comprise, for
example, other polymers, such as organic or inorganic polymer
particles and/or non-polymer, non-particulate property modifiers,
for example, viscosity modifiers, antioxidants, plasticizers, dyes
and the like. Polymer-inorganic particle composites can also
include polymers that are not bonded to the inorganic polymers or
crosslinked to the polymer bonded to the inorganic particles. These
non-bonded polymers may or may not have the same chemical
composition as the polymer bonded to the inorganic particles of the
composite. Similarly, a polymer-inorganic particle composite can
include non-bonded inorganic particles in addition to the bonded
inorganic particles. The non-bonded inorganic particles may or may
not have the same properties, such as composition, crystallinity,
average size and size distribution, as the bonded particles.
The other material(s) in a structure may or may not also be
polymer-inorganic particle blends. For example, the other materials
can be polymers or non-polymer inorganic materials. Specifically,
the polymer may or may not be the same polymer used in an adjacent
polymer-inorganic particle blend. Suitable polymers for
incorporating into structures include, for example, the polymers
that can be incorporated into the blends, as described above. These
polymers not formed into blends can be combined with additives,
such as viscosity modifiers, plasticizers, antioxidants, dyes and
the like. When polymers are placed adjacent polymer-inorganic
particle blends, the nature of the polymers and inorganic particles
in the adjacent layers generally determines the nature of the
interface. With respect to other materials at the interface,
suitable non-polymer inorganic materials include, for example,
elemental metals, metal alloys, metal/metalloid compositions,
carbon materials, such as graphite and amorphous carbon, and the
like. Non-polymer inorganic materials include crystalline and
amorphous compositions that are not covalently bonded into linear
polymer units. For the formation of optical structures, suitable
inorganic materials include, for example, TiO.sub.2, SiO.sub.2,
GeO.sub.2, Al.sub.2O.sub.3, P.sub.2O.sub.5, B.sub.2O.sub.3,
TeO.sub.2, and combinations, mixtures and doped versions thereof.
Non-linear optical materials, such as zinc oxide, KTaO.sub.3,
K(Ta,Nb)O.sub.3, YVO.sub.4, cadmium sulfide (CdS), cadmium selenide
(CdSe), indium phosphide (InP), lithium niobate (LiNbO.sub.3), and
barium titanate (BaTiO.sub.3), can be used within an optical
structure to modulate the wavelength, e.g., generate harmonics of
incident light, and/or for optical bistable or switch function as a
function of light power resulting from a non-linear power response,
which can be desirable in some optical devices. Dopants can be used
to increase the performance of the non-linear optical materials.
Suitable cadmium precursors for aerosol delivery into a laser
pyrolysis apparatus include, for example, CdCl.sub.2, and
Cd(NO.sub.3).sub.2, and suitable indium precursors for aerosol
delivery into a laser pyrolysis apparatus include, for example,
indium trichloride (InCl.sub.3). In addition, small crystalline
nanoparticles, e.g., no more than about 20 nm, can exhibit
non-linear properties due to imperfections that result in a loss of
inversion symmetry. These symmetry-breaking effects are enhanced
due to the small particle size. Thus, small nanoparticles, such as
crystalline silicon, can be used to take advantage of non-linear
optical effects. For optical applications, it is desirable to have
materials with large second order and/or third order electrical
susceptabilities (.chi..sup.(2) and .chi..sup.(3)) to obtain larger
optical non-linearity effects.
One or more materials within the structures can be an optical
material. In particular, one or more optical materials can be
incorporated within the structure such that the structure is an
optical structure. Optical structures can incorporate one or more
optical devices, that can be used to transmit and or manipulate
light within the structure. As used herein, an optical material
includes materials that can transmit light, with selected
wavelengths, with low loss due to scattering and absorption. In
particular, for transmission applications, optical materials have a
propogation loss at a particular wavelength in the infrared,
visible or ultraviolet of no more than about 20 percent over 1
centimeter, although desirable materials have significantly lower
propagation losses. Useful optical materials can be absorbing at
some wavelengths and transmitting at other wavelengths. For
example, amplifiers materials can absorb in ultraviolet and/or
visible and transmit in the visible or infrared. In other
embodiments, optical materials emit at desired frequencies upon
excitation by absorption or electrical stimulation. Thus, phosphors
and the like can be incorporated into polymer-inorganic particle
blends. Nanoparticle phosphors are described further in and
commonly assigned U.S. patent application Ser. No. 09/843,195, now
U.S. Pat. No. 6,692,660 to Kumar et al., entitled "High Luminescent
Phosphors," incorporated herein by reference. In some embodiments,
phosphors include a host crystal or matrix and a small amount of
activator. Suitable host materials for the formation of phosphors
include, for example, ZnO, ZnS, Zn.sub.2SiO.sub.4, SrS, YBO.sub.3,
Y.sub.2O.sub.3, Al.sub.2O.sub.3, Y.sub.3Al.sub.5O.sub.12 and
BaMgAl.sub.14O.sub.23. Generally, heavy metal ions or rare earth
ions are used as activators.
In particular, in some embodiments of interest, the
polymer-inorganic particle blend is an optical material. Generally,
the composition of the polymer and the inorganic particles are
selected appropriately to form an optical material with desired
optical properties. Similarly, the particle loadings are selected
to yield desired optical properties of the resulting blend. In
preferred embodiments, the blend is a composite to provide desired
amounts of stability at high particle loadings during processing
into desired structures.
The polymer-inorganic particle blends provide for continuous
selection of index-of-refraction over wide ranges. The
index-of-refraction of the blend generally is expected to be
approximately a linear combination by the weight ratios of the
index-of-refraction of the inorganic particles and the polymer. Any
non-linearities in the dependence of index-of-refraction as a
function of particle loadings can be accounted for empirically
based on measurements of the index-of-refraction. To form high
index-of-refraction materials, high particle loadings are generally
used with the inorganic particles correspondingly being selected
for a high index-of-refraction. Thus, the pure polymer generally
would provide the lower limit on the index-of-refraction for the
blend. The upper limit on the index-of-refraction for a blend as a
function of particle loading would be the index value at the
highest particle loadings. Specifically, TiO.sub.2 generally has a
high index-of-refraction with values ranging from about 2.5 to
about 2.9. InP and other phosphides, for example, have
indices-of-refraction greater than 3. SiO.sub.2 generally has a
relatively low index-of-refraction from about 1.45 to about 1.5.
Suitable polymers generally have a low index of refraction from
about 1.3 to about 1.6. Thus, polymer-inorganic particle blends can
be formed with an index-of-refraction up to 2.7 or more. With
interfaces between a polymer-inorganic particle blend and a polymer
or another polymer-inorganic particle blend, the differences in
index-of-refraction can be as small as desired or, in some
embodiments, from 0.001 to about 1.5 or more.
The use of nanoparticles within the blends has the advantage for
optical materials of higher transparency and reduced scattering of
light relative to optical properties of corresponding blends when
using larger inorganic particles. Nanoparticles are especially
effective in reducing scattering in the infrared portion of the
electromagnetic spectrum including wavelengths of about 0.8 microns
to about 5.0 microns. Thus, polymer-inorganic particle blends
formed with the nanoparticles will have correspondingly lower
scattering.
For embodiments in which the polymer-inorganic particle material is
an optical material, the adjacent material at the interface can
also be an optical material such that an optical interface is
formed. The other optical material at the interface can be a
polymer, a polymer-inorganic particle blend or a uniform inorganic
material. Polymers can be used especially to form a low
index-of-refraction material. Polymer-inorganic particle blends can
be used to incorporate desired index differences between the
materials at the interface and to incorporate desired optical
properties to the second material. Suitable uniform inorganic
materials include, for example, optical glasses, such as silica
glasses and doped silica glasses, and crystalline or
polycrystalline materials, such as quartz.
The difference in index-of-refraction between the two materials at
the interface generally is selected to form a desired device
incorporating the structure. In general, the difference in
index-of-refraction is at least about 0.0025, in other embodiments
at least about 0.005, and in further embodiments at least about
0.01. In some embodiments, relatively small differences are
sufficient to confine the light and to control the propagation
modes. In alternative embodiments, a larger difference in
index-of-refraction is used to obtain desired functionality. Thus,
it may be desirable to have a difference in value of the
index-of-refraction between the two materials at least about 0.05,
in further embodiments at least about 0.1, in other embodiments
from about 0.2 to about 2.5 and in further embodiments from about
0.5 to about 2.0. A person of ordinary skill in the art will
recognize that other values for the differences in value of
index-of-refraction between these explicit differences are
contemplated and are within the present disclosure.
The transition between two materials with different
indices-of-refraction can be formed with a gradual or continuous
change between the materials. The reflection is a function of the
difference in index-of-refraction between two materials at an
interface. The evaluation of transmission and reflection at an
interface between two optical materials with different
indices-of-refraction can be calculated using well known optical
formulas. Since the function is non-linear, i.e., quadratic, with
respect to the index-difference, the loss due to reflection at the
interface can be reduced or eliminated. This reduction in
reflection can be especially significant for transitions between
materials with a large difference in index-of-refraction between
the two materials.
The optical interfaces can be used to form optical devices with
simple or complex structures and/or functionalities. For example,
the polymer-inorganic particle blends can be used to form passive
optical devices such as waveguides/optical channels and
couplers/splitters and the like. The polymer-inorganic particle
blends can be used for the core and/or for the cladding of the
devices. The waveguides and the like, for example, can be within an
optical fiber or on a planar optical structure. Referring to FIG.
4, waveguide 140 has a core 142 surrounded by cladding 144.
Generally, core 142 has a higher index-of-refraction than cladding
144 such that light is confined within the core by total internal
reflection. The difference in index-of-refraction can be selected
to limit the modes of transmission of light at a particular
wavelength. If the core and cladding are both formed from
polymer-inorganic particle blends, the difference in
index-of-refraction can be tuned by selecting the particle loading,
composition of the polymer and/or composition and other properties
of the inorganic particles. Referring to FIGS. 5 and 6, a
coupler/splitter 146 is shown with a core material 148 surrounded
by cladding 150. Appropriate dimensions of the core orthogonal to
the propagation direction depend on the index-of-refraction and the
wavelength of light. Generally, however, the dimensions across the
cross section of a waveguide are within an order of magnitude of
the wavelength of light. Thus, for most optical applications, the
light channels have dimensions less than about 10 microns.
In planer embodiments of optical structures, waveguides and
comparable elements, such as those in FIGS. 4-6, have a layered
structure, generally on a substrate. In these embodiments, a
polymer-inorganic particle blend can be used as an over-cladding on
top of core and/or under-cladding of a uniform inorganic optical
material, such as a silica glass, to form an athermal waveguide. In
particular, the presence of the polymer-inorganic particle blend
can compensate for thermal stresses within the structure if the
index-of-refraction of the polymer-inorganic particle blend is
selected to account for index of refraction changes in the
structure due to the presence of thermal stress.
The polymer-inorganic particle blends and corresponding interfaces
with a second material can be incorporated into devices of
interest. Suitable devices can be optical devices. Devices that can
be formed with polymer-inorganic particle blends include, for
example, interconnects, reflectors, displays,
micro-electromechanical structures (MEMS), tunable filters and
optical switches. MEMS structures can be incorporated into optical
devices, for example, to adjust distances between components. Other
devices of interest have periodic variations in
index-of-refraction, as described below.
Low loss interconnects are depicted in FIGS. 7A and 7B. Referring
to FIG. 7A interconnect 151 comprises a core 152 within cladding
153. Core 152 connects a first, high index, material 154 with a
second, low index, material 155. Core 152 further comprises an
interconnect transition 156. Interconnect transition 156 comprises
a one or more layers 157 with indices-of-refraction intermediate
between the index of first material 154 and second material 155.
Layers 157 have a gradual, monotonic, transition in
index-of-refraction with a higher index-of-refraction toward first
material 154 and a lower index-of-refraction toward second material
155. The thickness and number of layers can be selected to reduce
the loss to arbitrarily small values due to reflection of light
transmitted between first material 154 and second material 155. By
making the layers smaller and increasing the number of layers,
interconnect transition can be made to approximate a continuous
transition in index with a loss approaching zero. Interconnect
transition 157 can be formed from polymer-inorganic particle
blends. The index can be changed conveniently in a step-wise way
or, alternatively, continuous way by altering the particle loading,
although the index can also be altered by changing the composition
of the polymer and/or inorganic particles. First material 154 and
second material 155 can each be an optical polymer, a
polymer-inorganic particle blends or a densified inorganic optical
material, such as a doped silicon oxide glass.
With a change in index-of-refraction, appropriate cross-sectional
areas of the core region of a waveguide, either fiber or planar,
can be altered without changing the propagation of the light. In
particular, with an increase in index-of-refraction, the core can
be made thinner and/or narrower. The interconnect can then
correspondingly change size gradually or step-wise along with the
change in index-of-refraction. Referring to FIG. 7B, interconnect
core 131 connects between high index core 133 and low index core
135. As shown in FIG. 7B, interconnect core 131 gradually tapers in
thickness from a thinner dimension adjacent high index core 133,
which is correspondingly thinner than low index core 133, to a
thicker dimension adjacent low index core 135. The index of
refraction can change in a continuous or step-wise way. Similarly,
in alternative embodiments, the thickness of interconnect core 131
can change in a step-wise way. Interconnect cladding 137, thinner
cladding 139 and thicker cladding 141 can be formed all from the
same material, with the same index-of-refraction, or from different
material, with correspondingly different indices-of-refraction.
Interconnect cladding 137 can change in thickness and/or
index-of-refraction in a step-wise or continuous way.
In addition, the polymer-inorganic particle blends can be used as a
glue between two other optical materials. The polymerization and/or
crosslinking can be completed following application of the blend as
an interconnect between two materials. Completion of the
polymerization/crosslinking can physically connect the two
materials and provide a continuous optical path. The
index-of-refraction of the polymer-inorganic particle blend can be
selected, as described herein, to approximately match the other
materials to reduce the loss. Due to high available particle
loadings the optical glue can have a high index-of-refraction.
Polymer alone or a lower index polymer-inorganic particle blend can
be added as a cladding that polymerizes or crosslinked to
completion following application to further assist with the
physical binding of the materials. Such an embodiment is shown in
FIG. 8. Core glue 161 and cladding glue 163 connect first optical
conduit 165 and second optical conduit 167. Core glue 161 comprises
polymer-inorganic particle blend.
Light is generally transmitted through waveguides due to total
internal reflection since the core has a higher index-of-refraction
than the surrounding cladding. Loss can occur due to bending of the
core if the index-difference between the core and cladding is not
large enough to confine the light within the core at the angles of
incidence at the bend. If the difference in index-of-refraction
between the core and the cladding is larger, sharper bends in the
core can be made without incurring loss in optical transmission.
The polymer-inorganic particle blends can be formed with a
relatively high index-of-refraction, as described above, such that
a larger difference in index between the core and cladding can be
formed. An appropriate degree of bending can be evaluated using
conventional optical formulas. Thus, bends with a greater angle can
be achieved relative to bends involving materials with a lower
achievable index-of-refraction. A bend is depicted in FIG. 9. Core
171 is surrounded by cladding 173. The angle at bend 175 can be
selected based on the difference in index-of-refraction to yield no
loss or an acceptably small loss. Reflectors/bends with sharper
angles can be used to form optical devices in a smaller
footprint.
Suitable displays include, for example, reflective-type displays.
In some embodiments, the polymer-inorganic particle blends can
replace conventional polymers within a polymer-dispersed liquid
crystal display. By selecting the desired index-of-refraction for
the polymer-inorganic particle blend, the index-of refraction can
be matched better to the adjacent materials such that less
undesirable reflection takes place. With less undesirable
reflection, the display element can have a sharper image. General
features of polymer-dispersed liquid crystal displays are described
further in U.S. Pat. No. 6,211,931 to Fukao et al., entitled
"Polymer-Dispersed Liquid Crystal Composition And Liquid Crystal
Display Elements Using The Composition," incorporated herein by
reference.
A portion of an embodiment of a polymer-dispersed liquid crystal
display is shown in FIG. 10. Display 160 has a first element 162
and a second element 164. Each element has a transparent electrode
166 and a transparent counter electrode 168 in a spaced apart
configuration. Transparent electrodes can be formed, for example,
from indium tin oxide. An outer transparent cover 170 covers the
viewing side of display 160. A black absorbing layer 172 is located
past transparent counter electrodes 168 on the side of display 160
opposite transparent cover 170. Polymer-inorganic particle blend
174 is located between transparent electrodes 166 and transparent
counter electrodes 168. Liquid crystal droplets 176 are dispersed
within blend 172. Liquid crystal droplets 176 can be microcapsules
or located within voids in the blend. Liquid crystal droplets can
include liquid crystals, such as cyanobiphenyl-based liquid
crystals (produced by Merck Corp.) and can include dyes.
Electrodes 166, 168 are connected to control circuitry 178 for the
selective application of an electric current to electrodes 166, 168
thereby generating an electric field between transparent electrodes
166 and transparent counter electrodes 168. When current is not
applied, the liquid crystals are randomly oriented, as shown in
element 162, such that incoming light through transparent cover 170
is scattered and element 162 has a color based on the dye. When an
electric field is applied, the liquid crystals align, as shown in
element 164, such that more light is transmitted to black absorbing
layer 172 and the element appears dark or off. Thus, when no
electricity is applied, the display looks white from reflection
from all the elements. If the polymer-inorganic particle blend has
an index-of-refraction closer to the index-of-refraction of the
liquid crystal droplets, greater amounts of light can be
transmitted to the black absorbing layer when electricity is
applied such that off elements are darker, i.e., the contrast
between on and off elements can be greater.
The polymer-inorganic particle blends can be incorporated into
micro-electromechanical systems (MEMS), especially for optical
applications, although non-optical applications are also
contemplated. Microelectro-mechanical systems for convenience will
be used generally to refer also to both micron scale systems and
submicron scale systems, nanoelectro-mechanical systems. MEMS
systems generally include a microactuator that can deflect in
response to applied stimuli, such as electric fields, magnetic
fields or thermal changes. For example, piezoelectric crystals
undergo a strain when an electric field is applied such that a
deformation related to the magnitude of electric field results.
Suitable piezoelectric materials include, for example, quartz,
barium titanate, lead zirconate-lead titanate and
polyvinylidenefluoride. Similarly, paramagnetic materials can be
used which can be designed to deflect in a magnetic field, such as
from an electromagnet. Thermal-based actuators can be formed from
interfaces between materials with differences in coefficients of
thermal expansion.
The polymer-inorganic particle blends can be incorporated into the
actuator element and/or into an extension from the actuator with
functional properties, such as desirable optical properties. In
particular, an element comprising a polymer-inorganic particle
blend can extend from a MEMS actuator, in which the element
functions as a mirror or a lens. An actuator-based optical element
can be incorporated into optical devices, such as a tunable filter
and or a tunable laser. These structures are discussed further in
the context of the following figures. With respect to incorporation
into the actuator element itself, the polymer and/or the inorganic
particles can be active with respect to actuator functionality.
Specifically, the polymer and/or the inorganic particles can be
piezoelectric materials, paramagnetic materials or materials with a
desired coefficient of thermal expansion. Similarly, the material
can have desired optical properties for incorporation into an
optical device as a movable optical element.
A tunable vertical cavity surface-emitting laser (VCSEL) is shown
in FIG. 11. Laser 190 includes a substrate 192 with a bottom mirror
194. Top mirror 196 is mounted on a membrane 198, which can be
formed from piezoelectric material. Membrane 198 is mounted on
posts 200. Electrode 202 is located along the top surface of
substrate 192. Membrane 198 and electrode 202 are connected to an
appropriate power source 204 and a variable resistance 206.
Membrane 198 and electrodes 202 form a portion of a MEMS device
incorporated into the tunable laser. The distance between mirrors
194, 196 determines the frequency of the laser. The position of top
mirror 196 is adjusted by providing a selective amount of current
using variable resistance 206. Since membrane 198 is formed from
piezoelectric material, membrane 198 deforms a particular amount
according to the amount of current applied. A pump pulse can be
supplied through top mirror 196 into the laser cavity. The
wavelength of the corresponding emissions through bottom mirror 194
depends on the position of top mirror 196. The lasing wavelengths
for a mode m is given by .lamda..sub.m=2nL/m, where n is the index
of refraction within the lasing cavity and L is the distance
between the mirrors. The peak gain is also a function of the
distance between the mirrors. The general structure of VCSELs is
described further, for example, in U.S. Pat. No. 6,160,830 to Kiely
et al., entitled "Semiconductor Laser Device And Method Of
Manufacture," incorporated herein by reference.
In tunable vertical cavity surface-emitting laser 190, one or more
components can be formed from the polymer-inorganic particle
blends. In particular, membrane 198 and/or top mirror 196 can be
formed from blends described herein. The index-of-refraction of top
mirror 196 can be selected to yield the desired optical properties.
Furthermore, membrane 198 can be formed from a polymer-inorganic
particle blend in which the polymer and/or the inorganic particles
have piezoelectric properties such that the application of current
to the electrodes can result in deformation of membrane 198.
Additionally or alternatively, one or more other components of
laser 190 can be formed from the blends.
Wavelength selective components are useful for performing
wavelength division multiplexing within networks to increase
bandwidth. Suitable dispersive elements include, for example,
diffraction gratings, prisms and the like. An arrayed waveguide
grating is another wavelength selective component of interest. An
arrayed waveguide grating comprises two couplers and an array of
waveguide channels, with one couple on each side of the array of
waveguide channels. The general principle of arrayed waveguide
gratings is described further in U.S. Pat. No. 5,002,350 to
Dragone, entitled "Optical Multiplexer/Demultiplexer," incorporated
herein by reference.
An embodiment of an arrayed waveguide grating is shown in FIG. 12.
Arrayed waveguide grating 210 comprises couplers 212, 214 and an
array of waveguides 216. Coupler 212 provides for coupling of an
input signal into array 216. At coupler 212, the waveguides of
array 216 are strongly coupled. Coupler 212 further is connected to
input waveguides 218. In one embodiment, coupler 212 can a physical
broadening of the respective waveguide cores of both array 216 and
input waveguides 218 and a gap between input waveguides 218 and
array waveguides 216 with uniform index-of-refraction such that
signals are coupled from the geometry. In some embodiments, coupler
212 includes broadened optical channels 220 that lead to each
waveguide of array 216.
Waveguide array 216 comprises a plurality of waveguides 226 with
different lengths from each other. The difference in lengths
results in a phase shift in the light signals transmitted through
the waveguides. The differences in lengths can be selected to
result in a desired interference between light of particular
wavelength at coupler 214. The interference at coupler 214 can
result in a spatial separation of light of different wavelengths.
The different wavelength of light can be directed to different
spatially displaced output waveguides 224.
In particular, waveguides of array 216 are also strongly coupled at
coupler 214. Coupler 214 can include broadened optical channels 222
that lead from waveguides of array 216. Broadened optical channels
222 can be arranged on an arc such that signals from each waveguide
interfere. Coupler 214 can further include spatially displaced
output waveguides 224 with the spatial separation being responsible
for a different frequency portion of the spectrum from the
interfering signal from array 216 transmitting through different
output waveguides 224.
While the embodiment in FIG. 12 has five waveguides shown, other
numbers of waveguides can be used. The number of waveguides and the
difference in lengths of the waveguides generally determine the
spectral resolution of the wavelength split signal. Active elements
can be incorporated into the arrayed waveguide grating to tune the
spectral decoupling. For example, an electroactive material and
corresponding electrodes can be introduced within one or more of
the waveguides of array 216. The use of active elements within an
arrayed waveguide grating is described further in U.S. Pat. No.
5,515,460 to Stone, entitled "Tunable Silicon Based Optical
Router," incorporated herein by reference. One or more of the
elements of the arrayed waveguide grating can comprise a
polymer-inorganic particle blend. For example, the core of the
waveguides of waveguide array can be a polymer-inorganic particle
blend. The index-of-refraction can be selected such that a
convenient path length difference is introduced into the waveguide
array.
Similarly, optical switches can be formed with polymer-inorganic
particle blends. A planar optical structure 230 with three optical
switches 232 is shown in FIGS. 13 and 14. Fewer optical switches,
additional optical switches and/or other integrated optical devices
can be incorporated into the structure, as desired. For
convenience, cladding layers are not shown. Optical switches 232
include cores 234 and switch elements 236. Switch elements 236
comprise a polymer-inorganic particle blend that is thermo-optical.
As shown in FIG. 13, the temperature of a switch element 236 is set
to adjust the index-of-refraction of the polymer-inorganic particle
blend such that the light in the waveguide is not transmitted and
the switch is closed. Generally, the temperature of a switch
element 236 can be selected to open and close the switch to control
light transmission through the switch. As shown in FIG. 14, a
thermal element 238, such as a resistive heater or a cooling
element, is placed near switch elements 236. Thermal element 238 is
used to control the temperature of adjacent switch element 236 to
open and close the switch by controlling the index-of-refraction of
switch element 236. The polymer-inorganic particle blend within
switch elements 236 can include polymer and/or inorganic particle
that are thermo-optical. Some polymers have a large negative change
in index-of-refraction in response to an increase in temperature.
Suitable polymers include, for example, halogenated polysiloxanes,
polyacrylates, polyimides and polycarbonates. Suitable
thermo-optical inorganic materials include, for example,
quartz.
A cross-connect optical switch is shown in FIG. 15. As shown in
FIG. 15, cross-connect optical switch 250 has two switch elements
252. Additional switch elements and/or integration with other
optical devices can be included in the structure, as desired.
Switch elements 250 can include thermo-optical, electro-optical or
magneto-optical materials. Appropriate electrodes, electromagnets
or thermal elements, as appropriate, can be properly placed
adjacent switch elements 252 to control the index-of-refraction of
switch elements 250. A light path strikes switch elements 250 at an
angle. The index-of-refraction of each switch element can be
selected to transmit or reflect most of the light as desired.
In some embodiments, structures have periodic formations
incorporating polymer-inorganic blends, such as polymer-inorganic
particle composites. The structure can have a periodicity in
composition and/or property, such as an optical property, in
one-dimension, two dimensions or three dimensions. Referring to
FIG. 16, structure 254 has a substrate 256 with four periodically
spaced bars 258 of polymer-inorganic particle blends. For
embodiments in which bars 258 are an optical material, periodically
spaced bars 258 result in a periodic variation in
index-of-refraction. Generally, bars 258 include the same materials
such that they have the same index-of-refraction as each other,
although alternative embodiments are described below. As shown in
FIG. 16, air or other gas fills the spaces between bars 258 as a
second material resulting in periodic interfaces between the
polymer-inorganic particle substrate and the gas.
In general, control of optical transmission is obtained by
embedding periodic optical materials within a larger structure.
Referring to FIG. 17, a three-layered optical structure 260 is
shown, in which hidden structure of the layers is shown with dashed
lines. Structure 260 includes a first layer 262, a second layer 264
and third layer 266. Second layer 264 includes periodically spaced
sections of optical material 268 comprising a polymer-inorganic
particle blend. Alternating optical material 270 is located between
periodic sections 268. Optical channels 272, 274, which are
outlined in dotted lines, extend in either direction from the
periodically spaced sections. Optical material 270 may or may not
be the same material as the optical material within optical
channels 272, 274. Optical channels 272, 274 are oriented
approximately along the axis normal to the periodicity of sections
268. Optical channels 272, 274 can function as a waveguide or the
like. The index-of-refraction of all of the materials can be
selected based on the desired function of periodic sections 268,
270 and optical channels 272, 274.
Referring to FIG. 18, optical structure 280 has a two-dimensional
periodic array 282 of alternating optical elements 284, 286. At
least, optical elements 284 comprise a polymer-inorganic particle
blend. As shown in FIG. 18, another condensed-phase optical
material 288, e.g., surrounding core material, is located between
around optical elements 284, 286. In some embodiments, optical
elements 286 and/or optical material 288 can be a gas. The optical
material of elements 286 can be the same as or different from
optical material 288. In some embodiments of interest, optical
structure 280 can be embedded within a larger superstructure with
additional materials and or features, such as structure 260 with
three layers 262, 264, 266 and optical channel 270. Similarly, an
optical structure 290 with three-dimensional periodicity is
depicted in FIG. 19. Optical structure 290 has a three dimensional
periodic array of optical elements 292, 294. At least optical
elements 292 comprise polymer-inorganic particle blends. Optical
elements 294 are located between and around optical elements 292
such that a periodic variation in index-of-refraction results in
three dimensions. Optical elements 294 can be formed from a
polymer-inorganic particle blend different from the material in
optical elements 292 or from a different type of optical material.
Optical elements 294 can be formed from the same optical material
as a surrounding optical material that integrates the periodic
optical structure into a larger optical structure in which the
period optical structure 290 is embedded.
For convenience, the period structures in FIGS. 16-19 are depicted
with three or four elements in the periodic structure. In further
embodiments, the number of elements in each dimension of the
periodic structure can be selected to obtain desired optical
effects. The optical effect of the periodic structure generally
depends on the optical properties of the material within the
periodic structure and in particular the difference in
index-of-refraction at the interface between the elements of the
periodic structure. In particular, optical properties of the period
structure depend on the index-of-refraction difference between the
periodic elements comprising a polymer-inorganic particle material
and the material between the elements comprising the
polymer-inorganic particle blend, although both of the alternating
optical material in the periodic structure can comprise
polymer-inorganic particle blends with adjacent elements having a
different composition and/or particle loading. In general, the
periodic structure has at least two elements in the period, in
further embodiments at least about 3 elements in the period, in
other embodiments at least 5 elements in the period, in further
embodiments from 2 elements to about 1000 elements, in additional
embodiments from 10 elements to about 250 elements and in still
other embodiments from 20 elements to 100 elements. A person of
ordinary skill in the art will recognize that other ranges within
these explicit ranges are contemplated and are within the present
disclosure. Generally, for some of the devices of interest, within
a structure with periodic variation in index-of-refraction, having
a greater difference in index-of-refraction at optical interfaces
results in a need for fewer periodic elements in the structure to
achieve a desired optical effect. The distance over which the
period repeats generally is at least about 10 nm, in further
embodiments at least about 20 nm, in other embodiments at least
about 50 nm, in additional embodiments from about 20 nm to about 10
microns and in other embodiments from about 50 nm to about 1
micron. A person of ordinary skill in the art will recognize that
additional ranges within these explicit ranges are contemplated and
are within the present disclosure.
As depicted in FIGS. 16-19, the index-of-refraction varies in a
step-wise fashion from one value to another value within the
periodic structure. However, by varying particle loadings and/or by
using different composition of inorganic particles, a continuous or
gradual step-wise change in index-of-refraction can be achieved.
Gradual step-wise variation in index-of-refract can have desirable
optical properties relative to step-wise variation between upper
and lower limits in index-of-refraction. Such versatility in index
selection can be used to approximate desired continuous functions
of index-of-refraction as a function of distance, for example, by
using a plurality of step-wise changes within the periodic
structure. In particular, it may be desirable to have approximately
a sinusoidal variation in index-of-refraction. Such a structure is
expected to give rise to a single reflection peak without the
presence of any higher order harmonics. A real space structure with
a refractive index profile made up of only a few (preferably a
single) harmonic will result in a reflection spectrum containing
only a few (or preferably a single) peak. Tuning the amplitude and
wavelength of the sinusoidal variation of refractive index
influences the strength of the light's interaction with the stack
(structure) as well as the wavelength of the light interacting with
the stack, respectively.
Referring to FIG. 20, periodic structure 296 has a periodic change
in index-of-refraction with step-wise changes in index. The
step-wise changes approximate sinusoidal variation in
index-of-refraction. The periodicity and the step-wise changes can
be seen in the plot of index-of-refraction (n) as a function of
distance "d" along the structure in FIG. 21. The number of steps in
each period and the number of periods can be selected to achieve
desired optical effects. Structure 296 can be incorporated into
larger superstructures as desired. In addition, similar step-wise
variation in index-of-refraction can be introduced within
two-dimensional and three-dimensional periodic structures.
Periodic variations in index-of-refraction within an optical
structure can be referred to as gratings (1-dimensional) or
photonic crystals (1-dimensional, 2-dimensional or 3-dimensional).
These devices can be used to form various optical devices. Thus,
the ability to form these periodic variations in
index-of-refraction provides a convenient approach to the formation
of integrated optical devices. Having an ability to select the
index-of-refraction differences provide increased flexibility in
device design. In particular, being able to form, using convenient
processing approaches, interfaces with increased differences in
index-of-refraction allows for the formation of smaller
devices.
Periodic index-variation in one-dimension can be used, for example,
to form Bragg gratings that have various optical applications. In
particular, Bragg gratings can be used, for example, to form
optical mirrors and optical band pass filters or interference
filters. In general, when light transmitted through an optical
material encounters a change in index-of-refraction, a portion of
the light is transmitted and a portion of the light is reflected.
If the variation in index-of-refraction is periodic, the relative
amounts of transmitted and reflected light depend on the difference
in index-of-refraction, the number of periodic elements and the
wavelength of light. Adjusting the difference in
index-of-refraction and the number of periodic elements can be used
to transmit and reflect desired portions of the spectrum. Mirrors
reflect desired portions of the spectrum. The gratings can be
incorporated into other structures such as lasers and the like.
Bragg gratings selectively transmit light wavelengths depending on
the number of grating elements and the index-of-refraction
differences between the elements of the grating. Bragg gratings
reflect some frequencies while transmitting other frequencies.
Polymer-inorganic particle blends can be used for one or more
components of the grating. By incorporating a blend with an
index-of-refraction that depends on electric field or temperature,
the filter can be made tunable. The relationships between
transmission and reflection wavelengths as a function of grating
parameters are described further in U.S. Pat. No. 6,278,817 to
Dong, entitled "Asymmetric Low Dispersion Bragg Grating Filter,"
incorporated herein by reference.
An embodiment of a tunable Bragg grating optical filter with
polymer-inorganic particle blends is shown in FIG. 22. Filter 310
includes three layers 312 of polymer-inorganic particle blend
interspersed with a low index material 314. The polymer-inorganic
particle blend in layers 312 function as an electro-optical
material in which the index-of-refraction varies with the
application of an electric field. Low index material 314 can be
air, a low index polymer, a low index polymer-inorganic particle
blend or other low index material. A transparent substrate 316 can
be used to support the filter, if desired. Spacers 318 can be used
to separate layers 312 in some embodiments. Electrodes 320, 322 can
be used to supply an electric field. Electrodes 320, 322 are
connected to power source 324, which can provide a variable voltage
to electrodes 320, 322 to provide desired tunability. In
particular, the index-of-refraction of layers 312 varies with the
application of an electric field while the filter function depends
on the index-of-refraction of layers 312. While the embodiment
shown in FIG. 19 has three alternating index-of-refraction elements
with high/low index, a greater number of elements in the grating
can be used to achieve desired filtering properties. A greater
index difference between the high index and low index components of
the grating results in a need for fewer grating elements within the
filter to obtain an equivalent resolution in the filtering.
With respect to polymer-inorganic particle blends in layers 312,
the polymer and/or the inorganic particles can have an
index-of-refraction that depends on electric field. Suitable
electro-optical inorganic materials comprise, for example,
lanthanum doped polycrystalline lead zirconate titanate, lithium
niobate (LiNbO.sub.3), KTaO.sub.3, LiTaO.sub.3, BaTiO.sub.3,
AgGaS.sub.2, ZnGeP.sub.2 and combinations thereof and doped
compositions thereof. Suitable electro-optical polymers include,
for example, polyimides with dissolved chromophores. Other
electro-optical polymers are discussed in U.S. Pat. No. 6,091,879
to Chan et al., entitled "Organic Photochromic Compositions And
Method For Fabrication Of Polymer Waveguides," incorporated herein
by reference. Similar tunability is obtainable with thermo-optical
materials within the polymer-inorganic particle blends if the
materials are correspondingly thermally controlled.
Lasers can be formed from two Bragg gratings that form the partial
mirrors of the laser cavity. Pump beams drive the laser. Such Bragg
grating lasers can be formed in optical fibers or as part of planar
optical structures. Lasers based on Bragg gratings are described
further, for example, in U.S. Pat. No. 5,237,576 to DiGiovanni et
al., entitled "Article Comprising An Optical Fiber Laser,"
incorporated herein by reference.
An embodiment of a laser formed with Bragg gratings is shown in
FIG. 23. Laser 340 comprises an optical channel 342 with Bragg
gratings 344, 346, amplification material 348 and secondary optical
pathway 350. Optical channel 342 generally is a core surrounded by
cladding in a fiber or in a planar structure. Bragg gratings 344,
346 have periodic variation in index-of-refraction and can
incorporate polymer-inorganic particle blends as described above.
The characteristics of the gratings can be selected to reflect and
transmit particular wavelengths for the laser function. Bragg
gratings 344, 346 form the boundaries of a laser cavity 352. The
size of the laser cavity determines the modes/wavelengths of the
laser emissions. Amplification material 348 is located within laser
cavity 352. Amplifier material 348 is optically connected to
secondary optical pathway 350. Amplifier material 348 includes
materials that absorb light at an amplifier wavelength with a
shorted wavelength than the wavelength of the laser. Generally, the
amplifier wavelength is in the ultraviolet. Suitable amplifier
material includes, for example, rare earth doped amorphous
particles. The production of rare earth doped amorphous particles
is described, for example, in copending and commonly assigned
Provisional Patent application 60/313,588 to Home et al., entitled
"Doped Glass Materials," incorporated herein by reference. These
particles can be incorporated into a polymer-inorganic particle
blend.
A pump beam is directed into the laser through waveguide 342. The
pump beam generally is in the visible or infrared portions of the
spectrum. A portion of the pump beam enters the laser cavity. An
amplification beam is direct to amplification material 348 through
secondary optical pathway 350. The amplification beam can be
supplied by an ultraviolet light source, such as an ultraviolet
laser or a non-laser light source. Energy from the amplification
beam is directed into the laser output by stimulated emission from
the amplification material due to the pump beam.
Lattices with periodic variation in index-of-refraction in one-,
two- or three-dimensions are referred to as photonic band gap
structures or photonic crystals. Photonic crystals have been
described as photonic analogs of electronic semiconductors.
Photonic crystals can provide a frequency gap covering a range of
frequencies of electromagnetic radiation that cannot propagate for
any wavevector, i.e., in any direction, including spontaneous
emission. Light can be introduced into a photonic crystal by
applying light at an angle to the periodic lattice. The frequency
gap depends on, for example, the unit cell size, the
crystallographic orientation of the periodic structure, the
indices-of-refraction including the differences in index between
different materials of the lattice and other optical properties. In
general, the differences in index-of-refraction between periodic
materials of a photonic crystal are at least about 0.1 index units,
in other embodiments, at least about 0.2 index units, in further
embodiments, at least about 0.5 index units, in additional
embodiments from about 0.2 to about 2 index units and in some other
embodiments from about 0.5 to about 1.5 index units. A person of
ordinary skill in the art will recognize that additional ranges
within these explicit ranges are contemplated and are within the
present disclosure. In general, the dimensions of the photonic
crystal lattice are on the same order of magnitude as the band gap
wavelengths.
Defects can be introduced into the photonic crystal to provide for
electromagnetic propagation within the forbidden band gap. The
defects introduce broken symmetry that interrupts the periodicity.
In particular, defects can be variations in the periodic structure
with respect to size, location and/or optical properties of an
element. Appropriate defects provide for selective propagation of
wavelengths. Defects can result from processing limitations with
respect to tolerances of the processing approach or they can be
purposely introduced. Thus, the photonic crystals with selected
defects can be used as optical filters, switches, amplifiers,
lasers and the like. In general, photonic crystals involve a
difference in index-of-refraction of two or more index units. The
evaluation of a photonic band gap for a one-dimensional photonic
crystal is given in U.S. Pat. No. 6,002,522 to Todori et al.,
entitled "Optical Functional Element Comprising Photonic Crystal,"
incorporated herein by reference. The polymer-inorganic particle
blends can be incorporated into periodic optical structures with
large variations in index-of-refraction in index-of-refraction
between the different materials of the lattice. Periodic optical
structures for forming photonic crystals can be formed as described
herein. Defects can be introduced by varying the periodic
structure. In particular, the periodic structures described above
can be used as photonic crystals within an optical structure if the
differences in index-of-refraction are sufficient.
The periodic structures, e.g., photonic crystals, can be used in
the formation of light absorbing structures, such as antenna and
solar cells. In these structures, a light absorbing electron donor,
such as a photoconductive polymer, such as a doped polyphenylene
vinylene can be placed adjacent the polymer-inorganic particle
blend that has a composition to function as an electron accepting
material. For example, the inorganic particles within the blend can
be fullerenes, other carbon nanoparticles or semiconductive
materials with electron holes for accepting the electrons, such as
micron sized silicon particles, as described further in U.S. Pat.
No. 5,413,226 to Matthews et al., entitled "Apparatus For Sorting
Objects According To Size," incorporated herein by reference.
Electrodes are placed around the electron donating and electron
accepting materials. If one surface is covered with an electrode,
the electrode can be a transparent electrode, for example, indium
tin oxide. Solar cell structures are described further in U.S. Pat.
No. 5,986,206 to Kambe et al., entitled "Solar Cells," incorporate
herein by reference.
In addition to periodic structures, optical structures can be
formed with polymer-inorganic particle blends that have
quasi-periodic or quasi crystalline structures. Quasi-crystal
structures can be quasi periodic in one-, two- or three-dimensions.
Quasi periodic one-dimensional optical structures are described in
U.S. Pat. No. 4,955,692 to Merlin et al., entitled "Quasi-Periodic
Layered Structures," incorporated herein by reference. In these
quasi periodic optical structures, layers of two different optical
materials with different indices-of-refraction can be ordered
according to a Fibonacci series with the following orderings, A;
AB; ABA; ABAAB; ABAABABA; ABAABABAABAAB, etc. A and B indicate
particular optical layered structures. The Fourier spectrum can be
evaluated for any of the resulting optical structures. For these
and other quasi-periodic structures, the optical performance of an
optical device can be modeled using existing computer modeling
techniques.
Processing of Composites into Structures
The polymer-inorganic particle blends generally can be processed
using methods developed for polymer processing. In selecting the
processing approaches for a blend, appropriate consideration can be
given to the physical properties of a particular blend as well as
the desired form of the resulting structure. Relevant physical
properties include, for example, viscosity, solubility, flow
temperatures and stability, although specific properties may only
be relevant for certain processing approaches. In particular, after
the formation of a polymer-inorganic particle blend, the blend can
be further processed for storage and/or for formation into desired
structures. The additional processing of the blend following its
formation may or may not take place in a solvent. The processing of
polymer-inorganic particle composites may be different from the
processing approaches for polymer-inorganic particle mixtures. In
particular, composites are more stable while the processing of
polymer-inorganic particle mixtures may need to maintain the
distribution of particles within the polymer. The processing of the
polymer-inorganic particle blends can be coordinated with
processing approaches for other materials for the formation of
interfaces and other components of structures into which the
polymer-inorganic particle blends are incorporated.
The blend can be molded, extruded, cast or otherwise processed
using polymer processing technology to form various shapes of
materials. In addition, the blend can be coated from a
solvent-based slurry, spin coated or the like to form a coating of
the composite. Any solvent can be removed following the formation
of a coating. Similarly, the blend can be crosslinked following
coating, whether or not a solvent/dispersant is used in the coating
process. Thus, the solidification process can involve
solvent/dispersant removal and/or crosslinking, such as thermal
crosslinking, crosslinking with ultraviolet light or an electron
beam, or by adding a radical initiator. The coatings can be
structured using mask techniques. In addition, self-assembly
techniques can take advantage of the properties of the components
of the composite to assist with the formation of structures on a
substrate, especially periodic structures, as described further
below. To the extent that self-assembly is used, the self-assembly
process is combined with a localization approach that overlays a
template as a boundary for the self-assembly approach.
Herein for convenience, the polymer-inorganic particle blend refers
to the bonded or unbonded inorganic particle and polymer/monomer
material whether in solution, a dispersion, a melt, a coating or a
solid form. For example, the properties of a solution/dispersion,
such as concentration and solvent composition, containing the
polymer-inorganic particle blend can be modified to facilitate the
further processing, for storage of the composite and/or for forming
structures. Solutions/dispersions that are more dilute generally
have a lower viscosity. In some processing approaches, the
polymer-inorganic particle composite is processed as a melt.
The solution/dispersion in which the composite is formed can be
used directly for further processing. Alternatively, the composite
can be removed from the liquid or placed in a different liquid. The
liquid of the solution/dispersion can be changed by dilution, i.e.,
the addition of a different liquid to solution/dispersion, by
dialysis to replace the liquid if the composite has sufficient
molecular weight to be retained by dialysis tubing, or by removing
the liquid and solubilizing/dispersing the composite with the
replacement liquid. Dialysis tubings with various pore sizes are
commercially available. To substitute liquids, a liquid mixture can
be formed, and subsequently the original liquid is removed by
evaporation, which can be particularly effective if the liquids
form an azeotrope. The polymer/inorganic composite can be removed
from a liquid by evaporating the liquid, by separating a dispersion
of the complex by filtration or centrifugation, or by changing the
properties, such as pH, liquid composition or ionic strength, of
the solution/dispersion to induce the settling of the complex from
the liquid.
Generally, the composite can be processed using standard polymer
processing techniques, including heat processing and solvent
processing approaches. For example, the polymer/inorganic particle
composite can be formed into structures by compression molding,
injection molding, extrusion and calendering. In other words, the
composites can be formed into free structures, such as sheets.
Similarly, the composites can be formed into fibers or a layer on a
fiber using techniques, such as extrusion or drawing a softened
form of the composite. Solutions/dispersions can be formed into
films/coatings by spin casting and similar methods. Coatings can be
formed with various parameters including, for examples, thin
coatings with thicknesses less than about 1 micron.
To form structures from the polymer-inorganic particle blends,
generally, the blends are processed along with one or more
additional materials to form appropriate interfaces within the
structures based on desired function. The processing of the
polymer-inorganic particle blends can further depend on the
properties of associated materials within the structures. Depending
on the desired structure, the polymer-inorganic particle blends may
or may not be localized within domains within a layer or other
extent of the structure.
For the formation of structures from the polymer-inorganic particle
blends, the blends can be selectively deposited over appropriate
regions or the blends can be selectively removed to leave the
desired structure. To selectively deposit the blend, the blends can
be deposited, for example, using print technology or using a
template. With respect to printing the blend, ink jet technology
can be adapted for the printing of the blends in an appropriate
solvent/dispersant along desired patterns.
With respect to template technology, standard lithographic
approaches using photoresist masks can be adapted for deposition of
the blends. For example, the blends can be deposited between gaps
in the photoresist. Excess blend outside of the gaps can be removed
along with the photoresist. Similarly, sacrifice layers can be
used. Sacrifice layers, like photoresist materials, are selectively
etched, for example, using a chemical compound, that selectively
removes the sacrifice layer after it has functioned as a template.
Alternatively, a physical mask can be used. A physical mask has a
separate structure apart from the surface, in contrast with
sacrifice layers and photoresist layers that are integral with the
surface being contoured. Physical masks can be physically removed
after the masking process is complete and, in some embodiments, can
be reused. Physical masks can be etched or cut, for example, with a
laser, from a ceramic material or a metal.
Similarly, a coating of polymer-inorganic particle blends can be
deposited, and a selected portion of the blend can be removed to
form a pattern. For, example, etching, such as dry etching, can be
used to selectively remove the blend. Reactive ion etching, such as
reactive oxygen etching, generally is appropriate for the removal
of polymer-inorganic particle blends. Lithographic techniques, such
as photolithography with photoresist, can be used to shield
portions of the blend during the etching process. Alternatively, a
focused ion or radiation beam can be used to perform the etching
without the need for a mask.
To form periodic structures, the polymer-inorganic particle blends
can be deposited using the above noted patterning approaches with a
periodic pattern. Alternatively, a period pattern can be formed by
taking advantage of self-assembly approaches to facilitate the
assembly process. Self-assembly processes take advantage of natural
ordering due to molecular ordering and/or molecular recognition.
The polymer-inorganic particle blends can exhibit self-organization
properties that can be exploited in self-assembly processes.
In particular, to facilitate formation into localized devices,
polymers can be selected for self-organization properties that
assist the self-assembly. The self-organization properties can be
associated with features of a copolymer or from a physical polymer
blend. Based on these potential self-organization properties of the
polymers, a polymer-inorganic particle blend can incorporate
self-assembly to form into a localized structure. Self-assembled
structures can be formed from self-assembly with particles
segregated to one or another phase of the polymer within the blend,
in which different polymer phases are identifiable due to
self-organization. In particular, some self-assembly operations
naturally form periodic structures that can be used in forming
periodic variations in index-of-refraction.
In addition, the formation of localized structures also involves
formation of boundaries for the structures. Generally, the
self-assembly process forms an ordered network while a localization
process forms the boundaries of the self-assembly process. Thus,
periodic structures can be formed with the self-assembly process
imposing the periodicity while a separate localization process
forms the boundary of the periodic structure.
As an example, ordered polymers have properties that can promote
natural segregation that can be exploited within a self-assembly
framework. Ordered polymers include, for example, block copolymers.
Block copolymers can be used such that the different blocks of the
polymer segregate, which is a standard property of many block
copolymers. Other ordered copolymers include, for example, graft
copolymers, comb copolymers, star-block copolymers, dendrimers,
mixtures thereof and the like. Ordered copolymers of all types can
be considered a polymer blend in which the polymer constituents are
chemically bonded to each other. Physical polymer blends may also
be used as ordered polymer and may also exhibit self-organization,
as described further in and commonly assigned U.S. patent
application Ser. No. 09/818,141, now U.S. Pat. No. 6,599,631 to
Kambe et al., entitled "Polymer-Inorganic Particle Composites,"
incorporated herein by reference. Physical polymer blends involve
mixtures of chemically distinct polymers.
Using ordered copolymers, a portion of the polymer-inorganic
particle blend can have a significantly different
index-of-refraction than another portion of the blend. Using
self-assembly techniques, the portions of the blend with different
indices-of-refraction can be ordered to form a physical interface
between the materials with the different indices-of-refraction.
Furthermore, periodic structures can be used to form periodic
variation in the index-of-refraction. Specifically, periodicity of
the index-of-refraction can be created in more than one dimension.
The one-dimensional and multidimensional variation in
index-of-refraction can be advantageously used to form photonic
crystals.
Suitable block copolymers for self-organization include, for
example, polystyrene-block-poly(methyl methacrylate),
polystyrene-block-polyacrylamide, polysiloxane-block-polyacrylate,
suitable mixtures thereof and the like. These block copolymers can
be modified to include appropriate functional groups to bond with
the linkers. For example and without limitation, polyacrylates can
be hydrolyzed or partly hydrolyzed to form carboxylic acid groups,
or acrylic acid moieties can be substituted for all or part of the
acrylated during polymer formation if the acid groups do not
interfere with the polymerization. Alternatively, the ester groups
in the acrylates can be substituted with ester bonds to diols or
amide bonds with diamines such that one of the functional groups
remains for bonding with a linker. Block copolymers with other
numbers of blocks and other types of polymer compositions can be
used.
The inorganic particles can be associated with only one of the
polymer compositions within the block such that the inorganic
particles are segregated together with that polymer composition
within the segregation block copolymer. For example, an AB di-block
copolymer can include inorganic particles only within block A.
Segregation of the inorganic particles can have functional
advantages with respect to taking advantage of the properties of
the inorganic particles. Similarly, tethered inorganic particles
can separate relative to the polymer by analogy to different blocks
of a block copolymer if the inorganic particles and the
corresponding polymers have different solvation properties. In
addition, the nanoparticles themselves can segregate relative to
the polymer to form a self-organized structure.
Polymer blends involve mixtures of chemically distinct polymers.
The inorganic particles may bond to only a subset of the polymer
species, as described above for block copolymers. Physical polymer
blends can exhibit self-organization similar to block copolymers.
The presence of the inorganic particles can sufficiently modify the
properties of the composite that the interaction of the polymer
with inorganic particles interacts physically with the other
polymer species differently than the native polymer alone. Even
with a single polymer, if the particles are not uniformly
distributed within the polymer, the polymer with higher particle
loadings can separate from the polymer portions with lower particle
loadings to form a self-assembled structure.
Regardless of the self-organization mechanism, some self-organized
polymer-inorganic particle blends involve particle, such as
nanoparticles, aligned with periodicity in a superstructure or
super crystal structure. The particles may or may not be
crystalline themselves yet they will exhibit properties due to the
ordered structure of the particles. Photonic crystals make use of
these crystal superstructures, as described further below.
The self-organization capabilities of a polymer-inorganic particle
blend can be used advantageously in the formation of self-assembled
structures on a substrate surface. To bind the composite to the
surface, the polymer can be simply coated onto the surface or the
composite can form chemical bonds with the surface. For example and
without limitation, the polymer can include additional functional
groups that bond to one or more structures and/or one or more
materials on the surface. These additional functional groups can be
functional side groups selected to assist with the self-assembly
process.
Alternatively, the substrate surface can have compositions, a
surface linker, that bond to the polymer and/or to the inorganic
particles such that a composite is bonded to the surface through
the polymer or the inorganic particles. For example, the substrate
can include organic compositions with one or more functional groups
such as halogens, such as Br, CN, SCOCH.sub.3, SCN, COOMe, OH,
COOH, SO.sub.3, COOCF.sub.3, olefinic sites, such as vinyl, amines,
thiol, phosphonates and suitable combinations of any two or more
thereof. In other embodiments, the surface linker has functional
groups that react with unreacted functional groups in the polymer.
Appropriate functional groups in the surface linker to bond with
the polymer can be equivalent to the functional groups in the
composite linker to bond with the polymer.
In some embodiments involving self-assembly with particles, such as
nanoparticles, a portion of the substrate surface is provided with
pores, which can be holes, depressions, cavities or the like. The
pores can be in an ordered array or a random arrangement. The size
of the pores should be larger than the size of the nanoparticles.
Generally, the pores have a diameter less than a micron, although
the preferred size of the pores and density of the pores may depend
on the particular desired properties of the resulting device. In
addition, the spacing between pores can be controlled to be on the
order of microns or submicron scales.
To deposit a polymer-inorganic particle blend within the pores, the
surface is contacted with a dispersion of the blend. Then, for
example, the dispersion is destabilized with respect to the blend,
such that the blend tends to settle onto the surface and into the
pores. The dispersion can be destabilized by altering the pH, such
as adjusting the pH toward the isoelectric point, by diluting
surfactants or by adding a cosolvent that results in a less stabile
dispersion. The dispersion is removed after the deposition of a
desirable amount of the blend. Then, blend on the surface not in
the pores can be removed. For example, the surface can be rinsed
gently with a dispersant to remove composite on the surface.
Alternatively, the surface can be planarized by polishing, such as
mechanical polishing or chemical-mechanical polishing. If the
dispersant is properly selected to be not too effective at
dispersing the blend and if the rinsing is not done too
extensively, the blend along the surface can be preferentially
removed while leaving the blend within the pores behind.
A porous structure can be formed using anodized aluminum oxide or
other metal oxides. Anodized aluminum oxide forms highly oriented
and very uniform pores. Pores are formed in anodic aluminum oxide
by place an aluminum anode in a solution of dilute acid, such as
sulfuric acid, phosphoric acid, or oxalic acid. As the aluminum is
oxidized, aluminum oxide with pores is formed. Pore diameters at
least can be varied between 4 nm and 200 nm. The pores have a depth
on a micron scale. The formation of porous anodized aluminum oxide
is described, for example, in D. Al-Mawlawi et al., "Nano-wires
formed in anodic oxide nanotemplates," J. Materials Research,
9:1014-1018 (1994) and D. Al-Mawlawi et al., "Electrochemical
fabrication of metal and semiconductor nano-wire arrays," in Proc.
Symp. Nanostructured Mater. Electrochem., 187th Meeting
Electrochem. Soc., Reno, Nev., May 21-26, 1995, Electrochem. Soc.
95(8):262-273 (1995), both of which are incorporated herein by
reference. The use of block co-polymers to form ordered array of
pores from silica and filling the pores to form a photonic crystal
is described in U.S. Pat. No. 6,139,626 to Norris et al., entitled
"Three-Dimensionally Patterned Materials and Methods For
Manufacturing Same Using Nanocrystals," incorporated herein by
reference.
The formation of a plurality of devices on a surface requires the
localization of compositions active in the devices within
prescribed boundaries associated with the particular device. To
localize a structure within prescribed boundaries by self-assembly,
the overall procedure generally requires both a process defining
the boundaries of the structure and a separate self-assembly
process using a chemical affinity to associate the compositions of
the device within the boundaries. The boundary defining process
generally utilizes external forces to define the extent of the
structures. The self-assembly process itself generally does not
define the boundaries of the structure. Self-assembly is based on a
natural sensing function of the compositions/materials that results
in a natural ordering within the resulting structure as the
compositions/materials associate. In general, the localization step
can be performed before or after the self-assembly process,
although the nature of the processing steps may dictate a
particular order. The net effect results in a self-assembled
structure with a corresponding coverage of polymer/inorganic
particle composite within the boundary and an area outside of the
boundary lacking this coverage.
The separate boundary defining process is coupled to the
self-assembly process by activating the self-assembly process
within the boundaries or by deactivating the area outside of the
boundaries. Generally, an outside force is applied to perform the
activation or deactivation process. The localization can be
performed, for example, using a mask or the like, or using maskless
lithography with focused radiation, such as an electron beam, an
ion beam or a light beam.
The identification of a suitable activation or deactivation
technique may depend on the particular self-assembly approach used.
The localization approaches generally involve either activation of
the area for the placement of the self-assembled structure or by
deactivating locations separate from the selected locations. In
particular, the localization approach isolates the region for the
formation of the self-assembled structure. Suitable physical forces
or chemical materials are applied to perform the
activation/deactivation.
Various approaches can be adapted for these purposes, including,
for example, conventional integrated electronic circuit processing
approaches. Specifically, mask techniques can be used to isolate
the boundaries of the activation/deactivation process. Radiation or
chemical application can be performed in regions defined by the
mask. Similarly, focused beams can be used to perform the
localization. Suitable focused beams to achieve surface
modification include, for example, light beams, such as ultraviolet
light or x-ray, laser beams, electron beams or ion beams, which can
be focused to impinge on the selected region to perform activation
or deactivation. Suitable focusing approaches are known in the
art.
An activation process can involve the formation of a specific
material at the desired location or the removal of a material or
composition that is inhibiting self-assembly at the desired
location. Specifically, a particular material can be formed within
the boundaries that allows for the self-assembly process to occur
within the boundaries, while the surface material outside of the
boundaries does not allow for the self-assembly process. For
example, a chemically reactive layer can be formed within the
boundaries that bind to a polymer, while the substrate surface
outside the boundary has a different chemical functionality that
does not bind to the polymer. Similarly, a layer of an inhibiting
compound can be removed from the area within the boundaries to
expose a surface material that binds to a compound required in the
self-assembly process, such as a surface linker. The inhibiting
compound can be a photoresist compound in some instances that
physically blocks the surface and is selectively removable before
or after the self-assembly process. The composition of the
photoresist or other inhibition compound is selected to inhibit the
self-assembly process such that the regions covered by the
inhibitory compound surrounding the boundary region subsequently do
not become involved in the self-assembly process.
Similarly, the regions outside of the boundary region can be
deactivated. For example, a composition that binds a compound
involved in the self-assembly process can be applied over an entire
surface. Then, the composition can be removed from outside of the
bounded region selected for the self-assembly process. Then, the
self-assembly process only takes place within the bounded region.
In addition, an inhibitor material can be specifically deposited
outside of the boundary region so that the self-assembly process
only takes place within the bounded region where the inhibitory
material has been removed. Similarly, radiation can be used to
inactivate or dissociate compounds outside of the bounded region.
The mask and/or focused beam approaches described above can be used
to perform the deactivation processes. As noted above, strata or
layers can be processed to produce a three-dimensional integrated
structure.
A localization process used along with self-assembly is described
further in and commonly assigned U.S. patent application Ser. No.
09/558,266, now U.S. Pat. No. 6,890,624 to Kambe et al., entitled
"Self Assembled Structures," incorporated herein by reference.
EXAMPLES
Example 1
Formation of Titanium Oxide Particles
Rutile TiO.sub.2, anatase TiO.sub.2, and oxygen deficient blue
TiO.sub.2 particles were produced by laser pyrolysis. The reaction
was carried out in a chamber comparable to the chamber shown in
FIGS. 24-26.
Referring to FIGS. 24-26, a pyrolysis reaction system 400 includes
reaction chamber 402, a particle collection system 404 and laser
406. Reaction chamber 402 includes reactant inlet 414 at the bottom
of reaction chamber 402 where reactant delivery system 408 connects
with reaction chamber 402. In this embodiment, the reactants are
delivered from the bottom of the reaction chamber while the
products are collected from the top of the reaction chamber.
Shielding gas conduits 416 are located on the front and back of
reactant inlet 414. Inert gas is delivered to shielding gas
conduits 416 through ports 418. The shielding gas conduits direct
shielding gas along the walls of reaction chamber 402 to inhibit
association of reactant gases or products with the walls.
Reaction chamber 402 is elongated along one dimension denoted in
FIG. 24 by "w". A laser beam path 420 enters the reaction chamber
through a window 422 displaced along a tube 424 from the main
chamber 426 and traverses the elongated direction of reaction
chamber 402. The laser beam passes through tube 428 and exits
window 430. In one preferred embodiment, tubes 424 and 428 displace
windows 422 and 430 about 11 inches from the main chamber. The
laser beam terminates at beam dump 432. In operation, the laser
beam intersects a reactant stream generated through reactant inlet
414.
The top of main chamber 426 opens into particle collection system
404. Particle collection system 404 includes outlet duct 434
connected to the top of main chamber 426 to receive the flow from
main chamber 426. Outlet duct 434 carries the product particles out
of the plane of the reactant stream to a cylindrical filter 436.
Filter 436 has a cap 438 on one end. The other end of filter 436 is
fastened to disc 440. Vent 442 is secured to the center of disc 440
to provide access to the center of filter 436. Vent 442 is attached
by way of ducts to a pump. Thus, product particles are trapped on
filter 436 by the flow from the reaction chamber 402 to the
pump.
Titanium tetrachloride (Strem Chemical, Inc., Newburyport, Mass.)
precursor vapor was carried into the reaction chamber by bubbling
Ar gas through TiCl.sub.4 liquid in a container at room
temperature. C.sub.2H.sub.4 gas was used as a laser absorbing gas,
and argon was used as an inert gas. O.sub.2 was used as the oxygen
source. Additional argon was added as an inert diluent gas. The
reactant gas mixture containing TiCl.sub.4, Ar, O.sub.2 and
C.sub.2H.sub.4 was introduced into the reactant gas nozzle for
injection into the reactant chamber.
Representative reaction conditions for the production of rutile
TiO.sub.2 particles and anatase TiO.sub.2 particles are described
in Table 1. The blue-oxygen deficient rutile TiO.sub.2
(TiO.sub.2-2) was obtained from the same conditions as the rutile
TiO.sub.2 particles (TiO.sub.2-1) in Table 1, except that they were
collected closer to the reaction zone by positioning the particle
collector accordingly. Low chamber pressure and low partial
pressure of oxygen contribute to the oxygen deficiency in the
resulting TiO.sub.2. Heating of the particles slightly in air
results in the loss of blue color and the formation of a rutile
structure. The reason for the color difference is not solely due to
level of oxygen content, and currently is not completely
understood.
TABLE-US-00001 TABLE 1 TiO.sub.2-1 TiO.sub.2-3 Rutile Anatase Phase
TiO.sub.2 TiO.sub.2 BET Surface Area (m.sup.2/g) 64 57 Pressure
(Torr) 110 150 Ar-Dilution Gas (slm) 4.2 8.4 Ar-Win (slm) 10.0 10.0
Ar-Sld. (slm) 2.8 2.8 Ethylene (slm) 1.62 1.25 Carrier Gas - Ar
(slm) 0.72 0.72 Oxygen (slm) 2.44 4.5 Laser Power - Input (Watts)
1400 1507 Laser Power - Out (watts) 1230 1350 sccm = standard cubic
centimeters per minute slm = standard liters per minute Argon -
Win. = argon flow through inlets 490, 492 Argon - Sld. = argon flow
through slots 554, 556
An x-ray diffractogram of product nanoparticles produced under the
conditions in Table 1 are shown in FIG. 27. Sample TiO.sub.2-1 had
an x-ray diffractogram corresponding to rutile TiO.sub.2. Sample
TiO.sub.2-2 had an x-ray diffractogram similar to sample
TiO.sub.2-1. Sample TiO.sub.2-3 had an x-ray diffractogram
corresponding to anatase TiO.sub.2. The broadness of the peaks in
FIG. 27 indicates that sample 1 is less crystalline than the other
two samples. Some peaks in the spectra of sample TiO.sub.2-1 seem
to originate from amorphous phases. Mixed phase particles can also
be produced. FIG. 28 represents a typical transmission electron
micrograph (TEM) of the particles. The average particle size
.phi..sub.av is around 10-20 nm. There are effectively no particles
beyond 2.phi..sub.av.
Optical absorption spectra were obtained for titanium oxide
particles in ethanol at a concentration of 0.003 weight percent.
The spectra for the TiO.sub.3-3 sample is shown in FIG. 29. For
comparison, similar spectra were obtained for a commercial
TiO.sub.2 powders dispersed in ethanol at a concentration of 0.0003
weight percent, which is shown in FIG. 30. The second commercial
powder was obtained from Aldrich Chemical Company, Milwaukee, Wis.,
and had an average particle size of 0.26 microns.
The absorption spectrum of the TiO.sub.2 in FIG. 30 is exemplary of
bulk TiO.sub.2 with a large absorption in the visible and infrared
portions of the spectra. In contrast, the absorption spectrum of
the powders in FIG. 29 has a very reduced absorption in the visible
and infrared portions of the spectra and enhanced absorption in the
ultraviolet. This shift and narrowing of the absorption spectra is
due to the reduced size of the particles.
Example 2
Nano-Polymer Composites
The formation of composites with poly(acrylic acid) and TiO.sub.2-3
powders with silane based linkers is described in this example.
The particles were well suspended in ethanol. Most of the particles
remained suspended after 2 weeks. High level of particle dispersion
was achieved, which was found significant for developing optical
quality nanocomposites. Secondary particle size in the suspensions
were evaluated with a Horiba Particle Size Analyzer (Horiba, Kyoto,
Japan). Analysis with the particle size analyzer showed good
dispersion/low agglomeration.
Surface treatment of the three types of TiO.sub.2 particles was
performed with aminopropyl triethoxy silane (APTES) as a silylation
reagent. APTES is thought to bond to the particles by the following
reaction:
Particle-Ti--OH+((CH.sub.3CH.sub.2O).sub.3--SiCH.sub.2CH.sub.2CH.sub.2NH.-
sub.2.fwdarw.Particle-Ti--O--Si(OCH.sub.2CH.sub.3).sub.2CH.sub.2CH.sub.2CH-
.sub.2NH.sub.2 Further successive hydrolysis of the ethoxy groups
can form additional Si bonds to the particle through ether-type
linkages. Some self-polymerization of the silylation reagent can
take place also, especially if excess silylation reagent and water
are present. Well-suspended APTES coated TiO.sub.2-3 particles were
prepared using ethanol as a solvent/dispersant.
Polyacrylic acid was added to the functionalized particles.
Generally, the polyacrylic acid had an average molecule weight of
250,000 Daltons, although some samples were prepared with low
molecular weight polymer having an average molecular weight of
2,000 Daltons. The polyacrylic acid is thought to react with by way
of the carboxylic acid group with the primary amine of the
silylation agent to form an amide bond. The first interaction of
the polymer with the surface treated particles involves the salt
formation of the carboxylic acid with the primary amine. Then, at
temperatures of 140.degree.-160.degree. the salt units condense to
form amide bonds. This reaction is depicted schematically as
follows: Polymer-COOH+H.sub.2N-- . . .
--Si--O--Ti-particle.fwdarw.Polymer-CONH-- . . .
Si--O--Ti-particle. A fourier transform infrared spectrum of the
composite had an infrared absorption band at 1664 cm.sup.-1, which
is a frequency characteristic of an amide bond. Scanning electron
microscopy (SEM) images confirm the successful synthesis of
TiO.sub.2-PAA nanocomposites. Also, the composites formed from the
functionalized particles exhibited significantly higher thermal
stability than corresponding poly-inorganic particle mixtures.
Coatings were formed of the resulting composite by placing drops on
a surface. The drops spread on the surface and were allowed to dry.
The dried composites were further analyzed. In particular, much
smoother materials were formed from the functionalized particles
(polymer-inorganic particle composites) than with the
unfunctionalized particles (polymer-inorganic particle
mixtures).
Example 3
Optical Measurements on PAA-Titania Composites
For the composites formed with polyacrylic acid and titania
particles, index-of-refraction as a function of particle loading
and optical loss were evaluated.
Refractive index measurements were performed using a Gaertner model
L-16C ellipsometer operating at 632.8 nm. Samples with different
levels of nanoparticle doping were coated on silicon wafers.
Refractive index measurements were performed at incidence angles of
50 and 70 degrees.
FIG. 31 illustrates evidence of index control that can cover a
substantial range based on selection of constituents and particle
loadings for polymer-inorganic particle blends, especially
composites. Solely by varying a loading level of TiO.sub.2
nanoparticles (n.sub.1.about.2.6-2.9 depending on anatase or
rutile) in a PAA host (n.sub.2.about.1.48), the index can be
controlled by a factor of over 150% with respect to that of the PAA
host. All index values are reference to light at 632.8 nm.
Appropriate selection of nanoparticles (high index) and polymer
host is expected to increase the controllable range of the
index.
Optical extinction measurements were performed using a Hewlett
Packard model 8452A spectrophotometer. Samples were suspended in
ethanol or prepared as films on fused silica substrates.
Measurements were performed in a fused silica cuvette with an
optical path length of 1 cm. Low optical loss was maintained over a
wide range of particle loadings. Even at a 50 weight percent
particle loading, the composites were found to have high levels of
transparency in the visible and infrared portions of the spectrum.
This observation is very significant with respect to application of
the composites as a building block for optical network
components.
FIGS. 29 and 30, respectively, show optical absorption spectra for
nano-TiO.sub.2 (.phi..sub.av.about.20 nm) and commercial TiO.sub.2
particles (.phi..sub.av.about.700 nm) both at a concentration of
0.003% by weight in ethanol. The latter scatters visible light far
more than the nanoparticles, thereby yielding a higher level of
optical attenuation. In addition, nano-TiO.sub.2 shows a
significant increase in the ultraviolet (UV) light absorption that
is considered as a quantum-size effect. This shift in absorption
spectrum can be used advantageously in optical materials for
transmitting visible or infrared light.
This example demonstrates a capability to control the refractive
index of nanoparticles-polymer composites through adjustment of the
particle loading. Use of preformed nanoparticles enables a large
index contrast between adjacent materials through adjustment of
particle loadings, although other materials changes can also be
used to establish a large index contrast at an interface with a
polymer-inorganic particle blend. A high level of uniformity in
nanoparticles as well as excellent dispersion and appropriate
surface modification over nanoparticles are useful for the
successful synthesis of photonic polymer-inorganic particle blends,
such as nanocomposites.
As utilized herein, the term "in the range(s)" or "between"
comprises the range defined by the values listed after the term "in
the range(s)" or "between", as well as any and all subranges
contained within such range, where each such subrange is defined as
having as a first endpoint any value in such range, and as a second
endpoint any value in such range that is greater than the first
endpoint and that is in such range.
The embodiments described above are intended to be illustrative and
not limiting. Additional embodiments are within the claims below.
Although the present invention has been described with reference to
specific embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention.
* * * * *